Marin Master Gardener: What's behind all that wilting?

Have you ever had a healthy strawberry plant wilt and die just as the fruit is starting to ripen? Have you ever wondered about the scattered die-back in a Japanese maple or why only half the avocado tree is dying? Perhaps you've had canes of raspberry plants die from the ground up? Maybe your tomatoes lose their vigor, display yellowing on older leaves or the fruit yield is just wimpy? If you have, there is a good chance you may have verticillium and it might be there to stay.

As with many fungal plant diseases, the three verticillium species have evolved fascinating structures and methods of infection, spread and especially survival. Verticillium is a "wilt" disease and causes plants to wilt by clogging up the vascular tissue of the plant, primarily the xylem — tube-like cell structures that bring water and nutrients up from the roots to the rest of the plant. Once infected, verticillium can move quickly through the xylem.

Spread of the disease from plant to plant usually occurs from root to root contact but verticillium can spread by air and water as well, traveling up to a mile from an infected plant. Seeds can also become infected and spread the disease even further.

The real problem is the ability of verticillium to stick around in the soil as mycelium for up to four years and microsclerotia for up to 15 years. Dormant structures such as these in the soil geminate when exposed to root exudates and go to work on even healthy plants.

There really is no way to cure an infection. Planting resistant plants and removing infected plants and plant tissue from the soil is the best defense. Crop rotation is a good second, and in particular, avoiding repeated plantings of solanaceous crops such as tomatoes, potatoes, eggplants or peppers. Both flooding and solarization can be helpful in some cases. Some root stocks are tolerant or resistant of the disease and some tomatoes have been engineered to be able to live with the disease.

While even healthy woody plants are susceptible, maintaining adequate irrigation and promoting new growth can help minimize the effects of the disease, which is favored by cooler weather.

Some good news: all monocots, ferns and conifers are immune. Corn, carrots, lettuce, beans, or peas are good plants to put in place where verticillium infection is suspected. Unfortunately, the list of susceptible plants is large and include common woody plants such as fraxinus, cinamonum, acer, olea, schinus, pistachea and yes, even rosa. Perennials such as dahlias, gerberas, marigolds, peonies, snapdragons and vinca are all susceptible.

The disease causes scattered wilting and branch dieback, leaving dark discolored vascular tissue on most species (not olives) and often affects only part of a plant.

Older leaves are infected first and the disease moves up the stem to infect newer leaves later. The disease wilts and turns a leaf's tissue between the veins yellow first; on tomato leaves, it forms a "V" shape..

A "V" on the seed packet or plant tag indicates resistance to the disease.
