10.6092/issn.2531-7342/13320
Status of Fusarium Head Blight (FHB) had thoroughly assessed on 52 soft wheat fields from 5 districts within 3 zones of Oromia, Ethiopia. The results showed FHB was 93.9% prevalent with significantly varied levels of incidences (among zones) and severity (among districts and zones). The highest incidence of 38.7 and 26.0% had been recorded in Buno-Bedele and Jimma zones. Correspondingly, the highest field-severity and FHB-index of 28.2 and 13.9% had been recorded in Buno-Bedele. Besides, the 2 mostly grown Danda’a and Digalu varieties were vulnerable to FHB sustaining 32.3 and 30.5% incidence, 21.8% and 21.7% field-severity, and 10.5% and 8.8% FHB-index. The variation in FHB intensity had mostly influenced by altitude, tillage frequency before sowing, and rainfall received during flowering to hard-dough stages. This study reveals evidence that FHB is becoming an important disease to wheat in Southwestern Ethiopia (SWE). Thus, demands an intervention to reduce its probable risk to wheat across SWE.
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Status of <em>Fusarium</em> head blight on wheat fields in Southwestern Ethiopia
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Ethiopian Institute of Agricultural Research, Kulumsa Agricultural Research Center, P.O. Box 489, Assela, Ethiopia
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Zadoks JC, Chang TT, Konzak CF (1974) A decimal code for the growth stages of cereals. Weed Research 14:415–421. https://doi.org/10.1111/j.1365-3180.1974.tb01084.x
Status of <em>Fusarium</em> head blight on wheat fields in Southwestern Ethiopia
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                IJM - Italian Journal of Mycology
                ISSN 2531-7342 - Vol. 50 (2021): 117-130
                Journal homepage: https://italianmycology.unibo.it/

Research article

                  Status of Fusarium head blight on wheat fields
                             in Southwestern Ethiopia

                       Minyahil Kebede1,2, Girma Adugna2, Bekele Hundie3

1
  Ethiopian Institute of Agricultural Research, Assosa Agricultural Research Center, P.O. Box 265, Assosa, Ethiopia
2
  Jimma University, College of Agriculture and Veterinary Medicine, Department of Horticulture and Plant Sciences,
P.O. Box 307, Jimma, Ethiopia
3
  Ethiopian Institute of Agricultural Research, Kulumsa Agricultural Research Center, P.O. Box 489, Assela, Ethiopia

Corresponding author e-mail: minishkebe@gmail.com


Article info
Received 20/7/2021; accepted 2/12/2021
https://doi.org/10.6092/issn.2531-7342/13320

Abstract
Status of Fusarium Head Blight (FHB) had thoroughly assessed on 52 soft wheat fields from 5 districts within 3 zones of
Oromia, Ethiopia. The results showed FHB was 93.9% prevalent with significantly varied levels of incidences (among
zones) and severity (among districts and zones). The highest incidence of 38.7 and 26.0% had been recorded in Buno-Be-
dele and Jimma zones. Correspondingly, the highest field-severity and FHB-index of 28.2 and 13.9% had been recorded
in Buno-Bedele. Besides, the 2 mostly grown Danda’a and Digalu varieties were vulnerable to FHB sustaining 32.3 and
30.5% incidence, 21.8% and 21.7% field-severity, and 10.5% and 8.8% FHB-index. The variation in FHB intensity had
mostly influenced by altitude, tillage frequency before sowing, and rainfall received during flowering to hard-dough sta-
ges. This study reveals evidence that FHB is becoming an important disease to wheat in Southwestern Ethiopia (SWE).
Thus, demands an intervention to reduce its probable risk to wheat across SWE.

Keywords
FHB, soft wheat, Triticum aestivum


Introduction

Wheat (Triticum spp.) is the second most cultivated cereal crop in the globe next to rice (FAO, 2018).
Ethiopia is the second-largest wheat producer next to Egypt in Africa (FAO, 2018) with a total of
4.54 million tons produced on 1.70 million hectares of land with a national average yield of 2.68 t
ha-1 (CSA, 2017) below the global yield of 3.65 t ha-1 (FAO, 2018). These are due to numerous factors
including biotic, abiotic, technical and socio-economic, and climatic factors (Barron et al., 2003; Liu
et al., 2008; Hailu et al., 2011; Mann and Warner, 2017). Micro- and macroorganisms were the main
constraints that caused crop losses in East Africa (Oerke, 2006). Among them, fungal pathogens like
Puccinia spp., Septoria spp., and Fusarium spp. on spike are the main constraints to wheat production
in Ethiopia (Hailu et al., 2011). Fusarium head blight (FHB) of wheat was one of the biotic stresses
that obtained the biggest concern (Tesfaye and Anisimova, 2016).
   FHB is the most destructive fungal disease of wheat worldwide, particularly in humid and semi-
humid wheat-growing areas (Martinez-Espinoza et al., 2014; Lenc, 2015). The disease is caused by
up to 19 species (Liddell, 2003). FHB pathogens infect several cereal crops including wheat, barley,
Department of Agricultural and Food Science in partnership with the Italian Mycological Society (UMI).
The journal is hosted and mantained by ABIS-AlmaDL. © 2021 The Authors. This work is licensed under CC BY 4.0
Kebede, Adugna, Hundie                                            Italian Journal of Mycology 50 (2021): 117-130

https://doi.org/10.6092/issn.2531-7342/13320



oats, rye, corn, canary seed, and forage grasses, but wheat, barley, and maize are the most affected
crops (Clear and Patrick, 2003; Kosová et al., 2009). Globally, the FHB of wheat has emerged as a
major threat to global food security (Goswami and Kistler, 2004; McMullen et al., 2012). Infection
of wheat kernels by FHB pathogens contributed to losses in grain yield and quality that includes poor
seed germination (or blighted seedlings), shriveled kernels, reduced number of kernels per spike, low
protein content in kernels, and low baking quality of wheat grains (Gärtner et al., 2008). In addition to
crop losses, several Fusarium species can produce a range of mycotoxins in infected grains, making
them unsuitable for animal and human consumption (Grabowski et al., 2012; Darwish et al., 2014).
    In sub-Saharan Africa (SSA), there is a lack of information regarding the FHB epidemics and
economic losses on wheat because of the underdeveloped research on the disease (Dweba et al.,
2017). Particularly in Ethiopia, there is little information on FHB of wheat that reported the disease
as one of the major wheat diseases at high altitude areas (Bekele, 1985), the 1988 cropping season
as a scabby season with an incidence of 85% and severity of 5–80% (Bekele, 1990), FHB incidence
of 47% in Ari district (Mitiku and Eshete, 2016), and mean FHB incidence and sevrity of 43.7%
and 76.3%, respectively in West Shewa Zone (Abdisssa and Bekele, 2020). Besides, the disease was
reported to cause yield losses of 60% and above under experimental conditions in the 1989 cropping
season of Ethiopia (Snijders, 1989). Recenltly, nine Fusarium spp. were identified to cause FHB on
soft wheat in Southwestern Ethiopia (SWE) (Kebede et al., 2020). In general, less concern was given
to the FHB of wheat in Ethiopia since then. In addition, the past FHB survey efforts were not enclose
SWE (particularly Jimma, Buno-Bedele, and West-Wollega zones) where wheat is grown as one of
the staple food crops. These three zones donated 4.89% of hectares and 3.52% of tonnes to the total
wheat production of the Oromia region, Ethiopia (CSA, 2017). Therefore, this study was aimed to
assess the occurrence and extent of FHB on wheat fields across SWE.

Materials and Methods

Description of study areas
This study was conducted across SWE namely in Dedo and Seka-Chekorssa districts of Jimma zone,
Bedele and Gechi districts of Buno-Bedele zone, and Begi district of West-Wollega zone as shown
in Fig. 1.




                      Fig. 1 - Surveyed zones and districts for FHB of wheat in SWE during
                      2017 cropping season.

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Sampling method and assessment of FHB disease
The purposive multistage sampling method had used to choose wheat-producing zones, districts
within zones, and “Kebeles” (Peasant Associations) within districts. Random sampling was applied
to select wheat fields in each “Kebeles”. Potential wheat-growing districts and “Kebeles” were chosen
by consulting zonal agricultural bureaus and the district’s agricultural and natural resource offices,
respectively.
    A field survey of FHB had carried out in 2017 during early milk to the hard dough wheat growth
stages (Zadoks et al., 1974). Within each field, disease assessment was made in four quadrats (30 ×
30 cm each) along the diagonal of the field at random. All wheat spikes within the quadrat had been
counted and visually examined for the presence or absence of FHB symptoms. Spikes were registered
as diseased when at least single spikelet had shown typical FHB symptoms (Fig. 2).

Collected data
The incidence of FHB was recorded as the percentage of infected wheat spikes (Wegulo et al., 2008)
per quadrat. Also, the severity of FHB had recorded on ten randomly chosen spikes per quadrat
following the modified Horsfall-Barrett’s scale (Stack and McMullen, 2011). The severity had then
partitioned to field severity (the average score of all assessed wheat spikes per field) and infected
head severity (the average score of only infected wheat spikes per field) (Stack and McMullen, 2011).
Moreover, the FHB index (an estimate of overall disease intensity) was determined from the product
of FHB incidence and field severity, divided by 100 (Wegulo et al., 2008). Finally, the prevalence
of FHB across SWE had determined as a proportion of wheat fields with FHB infection out of all
assessed wheat fields per district.
   Agronomic practices such as previous crops, altitude, fertilizer applied, weed infestation level,
source of seeds, wheat variety, planting pattern, and tillage frequency were recorded from each farm
to determine the relationship with incidence and severity of FHB of wheat.




                      Fig. 2 - A visual FHB severity rating scale in wheat expressed in % (Stack
                      and McMullen, 2011).



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Data analysis
The three-stage nested design procedure in SAS 9.3 statistical software (SAS, 2010) was used for
the analysis of FHB Incidence, field severity, infected head severity, and FHB index data. Means
were separated using the least significant difference (LSD) test at significance levels of 0.05. The
associations of disease intensity with altitude and previous crop were computed using the correlation
procedure of SAS 9.3 statistical software (SAS, 2010). The relationship of FHB incidence and field
severity (dependent variables) with independent variables was assessed using multiple regression
of SAS 9.3 (SAS, 2010). The three-stage nested model used in analyzing the data was described as
follows:
y ijk = ì + ôi + â j(i) + ã k(ij) + ål(ijk)
     Where: yijk is the FHB disease intensity where “Kebeles” k is nested within district J nested within
zone i, μ is the overall mean, τi is the effect of the ith zone, βj(i) is the effect of the jth district within the
ith zone, and γk(ij) is the effect of the kth “Kebeles” within the jth district and ith zone, and εl(ijk) is the error
term.

Results and Discussion

Occurrences and extent of FHB across SWE
Fusarium head blight disease of wheat was found widespread across all inspected districts in SWE
with an overall prevalence of 93.88%. At the district level, the disease was 100% prevalent in Seka-
Chekorssa, Bedele, and Gechi, 91.70% in Dedo, and 80% in Begi (Table 2).
    Analysis of variance showed that field severity and infected head severity of FHB had significantly
differed (at P < 0.01) among zones, districts within zones, and “Kebeles” within districts and zones.
In the same way, incidence and FHB index had shown a varying difference (at P < 0.01) among zones
and “Kebeles” within districts and zones (Table 2).
    The results indicated that FHB had registered varying incidences ranging from 0–100% in Jimma,
11.3–84.6% in Buno-Bedele, and 0–53.2% in West-Wollega zones during the 2017 main cropping
season of Ethiopia (Table 2). The average FHB incidence in wheat fields was 38.69%, 26.00%, and
13.82% in Buno-Bedele, Jimma, and West-Wollega zones of the Oromia region, respectively (Fig.
3). At the district level, the higher FHB incidences of 38.7% in Gechi, and 38.6% in Bedele districts
of the Buno-Bedele zone were recorded, followed by Dedo (26.6%) and Seka-Chekorssa (25.2%)
districts of Jimma zone. In contrast, the lowest incidence had recorded from the Begi (13.8%) district
of the West-Wollega zone (Table 2).
    According to a survey conducted in 1988 cropping season of Ethiopia, FHB incidence of 0 to 35%
was reported at farmer’s fields in Holeta and Kulumsa areas, 0 to 56% at experiment stations, 0 to
57% at seed production fields, and 0 to 84% at state farms (Bekele and Karr, 1997). Almost after 25
years, FHB disease of wheat had reported with an incidence of 10 to 47% at farmer’s fields during
2014 main cropping season in Ari district of South Omo Zone, SNNPR, Ethiopia (Mitiku and Eshete,
2016). Abdissa and Bekele (2020) reported that FHB incidence of 35–57% and severity of 52.8–
90.4% in West Shewa Zone, Oromia, Ethiopia. These showed an increasing trend in FHB incidence
over time in Ethiopia.
    As illustrated in Fig. 3, the highest severities of FHB had recorded in the Buno-Bedele zone with
field severity of 28.2%, infected head severity of 33.2%, and FHB index of 13.9%. These revealed
that FHB was more severe in the Buno-Bedele zone than Jimma and West-Wollega zones. On the
other hand, statistically similar field severity, infected head severity, and FHB index were recorded in
Jimma, and West-Wollega zones (Fig. 3). At district levels, the severity of FHB on wheat fields was
high in the Bedele and Gechi districts of the Buno-Bedele zone. The average field severity was 30.1%

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                 Fig. 3 - Mean incidence, field severity, and infected head severity of FHB during 2017
                 in three zones across SWE. Bars with a different letter for respective disease parameters
                 are significantly different at P < 0.01. LSD is the least significant difference.

in Bedele and 26.9% in Gechi districts of the Buno-Bedele zone. Also, higher severity of infected
heads had observed on wheat crops grown in Bedele and Gechi districts (Table 2).
    On the other hand, lower and statistically similar FHB field severities had been recorded in the
Seka-Chekorssa, and Dedo districts of the Jimma zone, and in the Begi district of the West-Wollega
zone (Table 2). Moreover, the FHB index ranging from 4.7 to 14.7% had been recorded on wheat
fields across SWE (Table 2). The disease was severe in the Gechi and Bedele districts of Buno-
Bedele, where the FHB index had been recorded with a magnitude of 14.7% and 12.6%, respectively
(Table 2). The use of fungicide application was suggested in North America when FHB severity is
above 10% (De Wolf et al., 2003). Also, in Brazil, a 7% severity in a group of spikes at the dough
stage has resulted in a significant reduction in kernel weight per spike, 1000 seed weight, and kernel
infection (Casa et al., 2004). In the current study, the average field severity of 12.8 to 30.1% at district
levels (Table 2) indicated that FHB disease of wheat is high in all the assessed zones that need an
intervention to minimize its probable risk on wheat in SWE.

Influence of agronomic practices on FHB of wheat in SWE
Results showed that FHB incidence was highly affected by the previous crop sown in the field, tillage
frequency before sowing the wheat crop, and the wheat varieties cultivated in the study area (Table 3).
In the same way, FHB field severities and infected head severities were also significantly influenced
by the previous crop sown in the field, tillage frequency before sowing the wheat crop, the grown
wheat varieties, and altitudes (agro-ecologies). Also, FHB field severity and infected head severity
was affected by weed infestation levels and sowing pattern, respectively (Table 3).
    As indicated in Table 3, seed sources such as AGP-II, Cascape project, local co-operative association,
and agricultural offices were found to contribute significantly to the occurrence and severity of FHB
disease of wheat across the three zones in SWE. The higher incidences had been obtained from wheat
fields sown by seed provided by AGP-II (47.7%), Cascape project (40.9%), and local co-operative
association (36.2%). Whereas higher FHB field severity and infected head severity were registered
in wheat fields sown by seeds provided by AGP-II (36.4%), and the Cascape project (27.9%). Also,
wheat seeds provided by agricultural offices had attributed to higher infected head severity in the
study area (Table 3). These might indicate the existence of varied infection levels of seed by FHB
pathogens. Seed-borne pathogens cause enormous crop losses by attacking seedlings, and kernel

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shriveling (Kubiak and Korbas, 1999; Gärtner et al., 2008). Therefore, the use of Fusarium-free
wheat seeds as a seed source prevents the entry of Fusarium inoculum into the wheat field.
   This study revealed that the preceding crops such as finger millet, wheat, maize, tef, and soybean
were positively correlated to the occurrence and severity of FHB disease of wheat in Jimma, Buno-
Bedele, and West-Wollega zones of Oromia region, Ethiopia. The higher incidences were recorded
from wheat fields previously planted by finger millet (53.2%), wheat (36.2%), and maize (35.7%),
followed by tef (29.1%), potato (24.01%), and soybean (14.2%). Also, higher field and infected head
severities had been recorded on wheat fields previously cultivated by finger millet, wheat, and maize
with values of 40.2% and 48.7%, 36.8% and 44.6%, and 26.3% and 34.1%, respectively (Table 3).
   Several studies in other Countries indicated that agronomic practices such as preceding crops, crop
variety, and tillage have an impact on the diversity and spread of Fusarium pathogens that caused
FHB on wheat (Dill-Macky, 2008; Fernandez et al., 2008; Katz, 2008; Wegulo et al., 2015).

Table 1 - Nested ANOVA table for mean squares incidence and severity of FHB.
 Source               Degree of freedom      FHB incidence           Field Severity      Infected head severity    FHB index
 Model                        47             2017.25**               1059.23**           1088.22**                 580.06**
 Zones                         2             8645.28**               5460.81**           3857.87**                 1703.43**
 Districts (Zones)             2             55.41ns                 662.33**            2012.06**                 37.28ns
 Error                        175            166.90                  85.10               165.70                    44.05
 Mean                                        28.47                   19.41               26.32                     9.04
 CV (%)                                      45.37                   47.53               48.91                     76.42
ns
     not significant at P < 0.05; ** significant at P < 0.01; ANOVA = Analysis of variance

Table 2 - Altitude ranges, prevalence (%), mean incidence (%) and severity (%) of wheat scab by districts in SWE, 2017.
 Districts        Altitude     Prev.      FHB incidence         Field severity       Infected head se-         FHB index
                  range of                                                           verity
                  fields                  Range        Mean     Range           Mean Range       Mean          Range       Mean

 Dedo             2328–2613        91.7   0.0–100.0    26.6b     0.0–59.7        12.3b    0.0–59.7    16.7c       0.0–       7.5ab
                                                                                                                  59.7
 Seka-Chekor- 2051–2462         100.0     7.6–44.2     25.2b     3.8–36.8        18.0b    10.7–44.6   27.3b       0.5–       5.9c
 ssa                                                                                                              13.3
 SubJimma     2051–2613            95.8   0.0–100.0    26.0      0.0–59.7        14.7     0.0–59.7     21.2       0.0–       6.8
                                                                                                                  59.7
 Bedele           1949–2009     100.0     18.0–69.8    38.6a    17.4–45.4        30.1a    22.4–50.4   37.3a       3.9–     12.6ab
                                                                                                                  22.5
 Gechi            2140–2269     100.0     11.3–84.6    38.7a     3.9–48.3        26.9a    9.6–49.0    30.6ab      0.4–       14.7a
                                                                                                                  38.2
 SubBuno-Bedele   1949–2269     100.0     11.3–84.6    38.7      3.9–48.3        28.2     9.6–50.4     33.2       0.4–       13.9
                                                                                                                  38.2
 Begi             1711–1951        80.0   0.0–53.2     13.8c     0.0–41.6        12.8b    0.0–53.7    24.4bc      0.0–       4.7c
                                                                                                                  21.3
 Overall          1711–2613        93.9   0.0–100.0    28.5                      19.4                  26.3                  9.0
 LSD                                                   10.3                      7.2                   8.2                   5.7
Mean values in a column with different letters are significant at P < 0.05; CV = coefficient of variation; Prev. = prevalence


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Kebede, Adugna, Hundie                                                                                                                  Italian Journal of Mycology 50 (2021): 117- 130

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   Table 3 - The effect of altitude and agronomic practices on the mean intensity of FHB during 2017 on soft wheat fields in SWE.
    Agronomic                           Proportion of     DIa        FSb                                                                     Proportion      DI        FS      IHS
                   Class                                                        IHSc (%)    Agronomic practices             Class
    practices                           fields (%)        (%)        (%)                                                                     of fields (%)   (%)       (%)      (%)
                   Barley               5.8               5.9cd      2.1d       7.6e                                        1711–2269        73.1            29.6      22.8a   32.2a
                   Faba bean            5.8               11.0cd     6.2cd      12.3de      Altitude ranges of assessed     2328–2613        26.9            25.5      11.8b   17.6b
                                                                                            fields
                   Finger millet        7.7               53.2a      40.2a      48.7a                                       LSD                              NS        8.9     9.7
                   Field pea            3.9               12.6cd     9.2cd      16.7cde                                     July             48.1            31.0      17.7    24.6
    Previous        Maize               19.2             36.9   ab
                                                                    26.9   ab
                                                                              34.3  abc   Planting date                    August            51.9             22.9     19.5     29.2
    crop            Potato              3.9              24.0bc 5.1cd         11.9de                                       LSD                                NS       NS       NS
                    Sorghum             5.8              0.0d       0.0d      0.0e                                         Low               57.7             24.3     19.0     28.5a
                    Soybean             3.9              14.3  bcd
                                                                    12.8  bcd
                                                                              42.7 ab
                                                                                                                           Medium            30.8             34.1     21.2     27.9ab
                                                                                          Weed infestation j
                    Tef                 28.9             28.9  bc
                                                                    19.4  bc
                                                                              28.0 bcd
                                                                                                                           High              11.5             19.7     9.9      16.4b
                    Wheat               15.4             36.2  ab
                                                                    36.8  a
                                                                              44.6 ab
                                                                                                                           LSD                                NS       NS       11.6
                    LSD                                  23.9       16.4      18.8                                         AGP-II d          13.5             47.7a    36.4a 40.8a
                    2 times             9.6              5.0c       4.1b      14.0b                                        AgrOff e          36.5             25.4bc 20.7b 31.1ab
     Tillage        3 times             26.9             22.4  b
                                                                    18.5  a
                                                                              29.8 a
                                                                                                                           AsARC     f
                                                                                                                                             13.5             2.8d     2.7d     14.9cd
     frequency
                    4 times             28.9             31.0  ab
                                                                    21.2  a
                                                                              26.2 a
                                                                                          Source of Seed                   Cascape-JU    g
                                                                                                                                             9.6              40.9 ab
                                                                                                                                                                       27.9  ab
                                                                                                                                                                                33.1ab
     before of
     sowing         5 times             34.6             36.2  a
                                                                    21.6  a
                                                                              26.2 a
                                                                                                                           Farm saved        13.5             14.2 cd
                                                                                                                                                                       4.3 cd
                                                                                                                                                                                9.3d
                    LSD                                  9.4        6.4       7.4                                          LC  h
                                                                                                                                             13.5             36.2 ab
                                                                                                                                                                       18.9  bc
                                                                                                                                                                                25.1bc
                    Row                 73.1             28.5       21.2  a
                                                                              31.4 a
                                                                                                                           LSD                                20.2     14.7     14.7
     Sowing
                    Broadcast           26.9             22.8       12.1b     19.6b                                        Unfertilized      34.6             22.0     16.1     26.1
     pattern                                                                              Fertilizer application
                    LSD                                  NS         8.4       9.1                                          25–50             25.0             32.6     19.1     25.0
                                                                                          (Kg ha-1)
                    Danda’a             50.0             30.5  ab
                                                                    21.7  b
                                                                              30.6 a
                                                                                                                           100               40.4             27.3     20.5     28.8
                    Digalu              25.0             32.3ab 21.8b         28.9a                                        LSD                                NS       NS       NS
     Wheat          Kakaba              6.3              15.2c      18.0bc    36.2a
     varieties      Kubsa               6.3              43.1a      32.7a     39.0a
                    Triticale           12.5             14.2  c
                                                                    4.3 c
                                                                              9.3b
                    LSD                                  13.0       8.9       10.5
   Mean values with the same letter within a column did not significantly differ at P < 0.05; a DI is disease incidence; b FS is field severity; c IHS is infected head severity; d The
   second agricultural growth program of Ethiopia; e Agricultural offices of the respective districts; f Assosa agricultural research Centre; g Cascape projected implemented by
   Jimma University; h local cooperative in Dedo district; i areas with 1500–2300 is cool sub-humid (Woina-dega) and 2300–3200 cool and humid (Dega) (MoA, 1998); j Weed
   infestation was recorded as low (for low weed infestation); medium (moderate weed infestation) and high (no weeded fields).
Kebede, Adugna, Hundie                                        Italian Journal of Mycology 50 (2021): 117-130

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Wheat and maize are the main hosts for Fusarium spp. that cause FHB in wheat (Kuhnem et al.,
2015). Also, Fusarium spp. such as Fusarium graminearum, F. culmorum, F. verticillioides, F.
sporotrichoides, F. oxysporum, and F. solani were reported as seed-borne pathogens of finger millet
in India, that confirmed finger millet as a host for FHB pathogens (Penugonda et al., 2010; Sobha-
Rani and Dorcas, 2016). Besides, the soil-borne Fusarium spp. (F. poae, and other 21 Fusarium spp.)
were also pathogenic on finger millet seedlings in Nigeria (Akanmu et al., 2013). The pathogenic
Fusarium species responsible for FHB on wheat across the study area can survive saprophytically on
finger millet residues, possibly explaining the higher FHB infection been recorded on wheat fields
previously planted by finger millet in the study area.
    Maize (corn) as preceding crop, cereal after cereal cropping, and zero or minimal soil tillage were
reported to favor the spread of Fusarium infection on cereals (Fernandez et al., 2008; Wegulo et al.,
2015). A greater FHB severity had been reported in wheat fields directly sown on corn residues when
compared to those planted on soybean residue (Dill-Macky and Jones, 2000). These can explain that
preceding crops play a great role in promoting FHB severity on the succeeding wheat crop either by
being a suitable host which increasing the inoculum levels within the field or by producing bulky crop
debris suitable for the saprophytic survival (Dill-Macky and Jones, 2000; Beyer et al., 2007).
    According to a study conducted in Uruguay, higher colonization of Fusarium species had been observed
on residues of wheat and barley than maize, but maize residues can be a source of primary inoculum for
three years (Pereyra and Dill-Macky, 2008). Remarkably, all the Fusarium species that cause FHB disease
are capable of surviving as saprophytes (Parry et al., 1995) on a range of crop residues including corn,
small grain cereals, and numerous other grass species and become a primary source of inoculum for FHB
disease of wheat (Keller et al., 2003; Dill-Macky, 2008; Pereyra and Dill-Macky, 2008).
    A study on the effect of crop rotation on FHB development reported a higher FHB incidence
(23.8%) in soft winter wheat planted following corn-soybean rotation, as compared to the lower
incidence (0.9–6.0%) in soft winter wheat planted after corn-pea rotation (Del Ponte et al., 2003).
This indicate that F. graminearum (the dominant FHB pathogen in SWE) can survive on stubbles of
field pea that would likely become a potential inoculum source reservoir for the succeeding soft wheat
crops (Chongo et al., 2001). Indeed, soft wheat fields previously grown by field pea were showed
considerable severity of FHB in SWE.
    Besides, viable spores originated from windblown infected residues of wheat or other hosts from one
cereal field to the next (Phalip et al., 2006), transportation of infected crop residues and seeds to new
areas (Government of Alberta, 2018), and windblown viable aerial ascospores at lower atmosphere up to
182.88 m above ground from distant (over kilometers) that can settle through precipitation or gravitation
(Maldonado-Ramirez et al., 2005) may act as a primary source of inoculum for wheat infection (Keller
et al., 2014). As a result, wheat plants in fields without cereal residue may also develop FHB disease.
    This study found that wheat variety Kubsa was highly infected by FHB having a mean incidence,
infected head severity, field severity, and FHB index of 43.1%, 39.0%, 32.7%, and 14.0%, respectively.
After that, variety Kakaba had shown higher infected head severity of 36.2%, but it had a moderate
field severity of 18.0% (Table 3). Digalu and Danda’a varieties had FHB infections of 32.3% and
30.5% incidence, 21.8% and 21.7% field severities, 28.9% and 30.6% infected head severities, and
10.5% and 8.8% FHB index, respectively (Table 3). Similarly, higher severity of 40.1% had previously
been reported on Danda’a variety in the South Omo zone, Southern Nation Nationalities and People
(SNNP), Ethiopia (Mitiku and Eshete, 2016).
    This investigation found that all the released bread wheat varieties grown in SWE have sustained
FHB index ranged from 5.0% to 14.0% (Table 3), while the highest and statistically similar FHB index
of 14.0%, 10.49%, and 8.8% was recorded for Kubsa, Digalu, and Danda’a varieties, respectively
(Table 3). Similarly, Abdissa and Bekele (2020) reported that all the improved varieties under
production (including Kingbird, Danda’a, Kakaba, Kubsa, and Digalu) were susceptible to FHB.
These results indicate that the two most popularly grown wheat varieties (Danda’a and Digalu) were

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susceptible to FHB disease of wheat like Kubsa variety, suggesting that FHB could pose a major
threat to wheat production across SWE in case an epidemic of FHB could occur. Therefore, it is of
great importance the screening for FHB resistant wheat varieties across all available wheat genotypes
for Ethiopia to reduce the risk of the disease development in the country.
    In contrast, lower mean incidence (14.2%), field severity (4.3%), infected head severity (9.3%), and
FHB index (0.8%) had been recorded on Triticale cultivar across SWE (Table 3). Triticale cultivars
found across SWE can grow more than 150 cm tall. According to the study conducted during 2013
and 2014 in Ottawa (Ontario, Canada), taller eastern spring wheat varieties showed a strong negative
relationship with the FHB index, meaning that the spikes of the taller plants were less prone to FHB
infection. Furthermore, the microclimate of taller plants is less favorable to FHB disease due to lower
relative humidity (Moidu et al., 2015), thus explaining why also in SWE Triticale is less vulnerable to
FHB infection. Additional investigation is needed to confirm whether plant height or other traits can
explain the lower FHB infection on Triticale.
    In SWE, higher FHB incidences and severities were recorded from wheat fields tilled 3 to 5 times by
the traditional ox traction system ahead of wheat seeding (Table 3). The traditional land tillage practiced in
Ethiopia does not bury all the left-over crop residues into the soil. Also, farmers usually left crop residues
in the field as it is, or collect them together in small humps within the field. Those left-over remnants may
serve for the saprophytic survival of FHB pathogens, which could be a source of primary inoculum for
the following cropping season. Even the more intensive tractor plowing did not decrease FHB incidence
as compared to conventional tillage, reasonably due to the returning effect of buried crop residues onto
the soil surface (Lenc, 2015). To overcome this, inverted tillage (moldboard) for land preparation had
recommended to buried the Fusarium-infected crop residues deep into the soil in developed countries
(Dill-Macky and Jones, 2000; Pereyra et al., 2004; Lenc, 2015; Hofgaard et al., 2016).

Relationship of FHB incidence and severity with independent variables
Analysis of multivariate linear regression revealed that 28.3% variability in incidence and 31.3%
variability in field severity had been explained by nine explanatory variables (Table 4).
Table 4 - Multiple regressions for wheat scab incidence and severity on independent variables in SWE, 2017 main
cropping season.
                                            FHB incidence                             FHB field severity
  Independent variables
                             Coefficients    Type-III SS P-values           Coefficients Type-III SS P-value
  Intercept                  -13.26          -              -               -23.46         -              -
  Altitude (m)               11.90           448.14         0.317           14.86          698.17         0.042
  Wheat variety              0.49            6.87           0.901           -1.24          44.69          0.657
  Source of seed             -1.78           99.59          0.635           -0.25          1.98           0.925
  Frequency of tillage       9.30            2878.02        0.014           6.13           1248.37        0.023
  Sowing date                -2.28           42.07          0.758           4.65           175.24         0.381
  Sowing pattern             -4.51           104.97         0.626           -2.23          25.68          0.736
  Previous crop              2.14            364.40         0.366           0.63           31.53          0.709
  Weed infestation           -6.69           750.18         0.197           -6.62          735.28         0.077
  Fertilizer application     0.06            304.08         0.409           0.03           88.50          0.533
  R2                                         28.3%                                         31.3%
  Adj. R2                                    11.8%                                         15.4%
  Pr> F                                      0.118                                         0.069
  Intercept                  -5.60                                          -4.08
  Rainfall a
                             0.044           3619.35        0.006           0.03           1773.28        0.008
  R2                                         15.3%                                         14.0%
  Adj. R2                                    13.4%                                         12.2%
a
 The total rainfall from August to November 2017 during the period when wheat was flowering to hard dough stages; SS
= sum of squares
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   The information brought by most of the explanatory variables was not significant for both
incidence and field severity (Table 4). However, the frequency of tillage ahead of seeding wheat
brings significant information to explain the variability in FHB incidence and field severity. Similarly,
differences in altitude can explain the variability in field severity across SWE. Furthermore, 15.3% of
the variability in incidence and 14.0% of the variability in field severity were significantly correlated
with (at P < 0.05) the total rainfall that occurred from wheat flowering to hard dough stages (Table
4). Therefore, the frequency of tillage ahead of seeding wheat, altitude, and the total rainfall that
occurred during flowering to hard-dough stages were the most influential variable in explaining the
variability of FHB in SWE (Table 4).
   The risk of FHB disease is correlated with the prevailing weather conditions such as warm and
humid conditions, during flowering (Xu, 2003; Popovski and Celar, 2013). The frequent rainfall,
high humidity, and heavy dew during flowering to soft-dough stages favour FHB infection in wheat
(Osborne and Stein, 2007; Popovski and Celar, 2013; Nazari et al., 2018; Schöneberg et al., 2018).
In SWE, the temperature during the flowering to soft dough stages of wheat growth (from August
to November 2017), ranged from 11.8 to 25.9 °C, and the precipitation ranged from 40.68 to 323.58
mm. These conditions were favourable for the infection of wheat spikes.

Conclusions

Fusarium head blight of wheat is becoming an important fungal disease of improved soft wheat
varieties across SWE as compared to the Triticale variety, particularly in the Buno-Bedele zone of
the Oromia region. In SWE, the incidence and field severity of FHB on wheat was mostly influenced
by the tillage frequency ahead of seeding wheat, altitude, and the total rainfall that occurred during
flowering to the hard dough stages. Therefore, the management of FHB of wheat across SWE should
be focused on the use of Fusarium free wheat seed, fungicide efficacy evaluation, and screening
of bread wheat genotypes for FHB resistance. Also, the evaluation of the incidence of the several
Fusarium species associated to FHB and the levels of their mycotoxins should be carefully monitored.

Declaration of interest
The authors have not declared any conflict of interest.

Acknowledgments
The authors are thankful to the Ethiopian Institute of Agricultural Research and the Assosa Agricultural
Research Center for the financial, vehicle, and material support. We also extend an appreciation to
Mr. Mathiwos Teshome and Mr. Chala Edao for his valuable assistance in translating “Afan Oromo”
during the survey, and Mr. Adisu Aragaw (a driver) for his nice transportation from place to place.

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