 Although pyrethrum (natural pyrethrins) is more effective against insects when used with piperonyl butoxide (which retards degradation of the pyrethrins), organic standards generally do not permit use of the latter substance.Naturally derived fungicides allowed for use on organic farms include the bacteria "Bacillus subtilis" and "Bacillus pumilus"; and the fungus "Trichoderma harzianum".These are mainly effective for diseases affecting roots.