Verticillium wilt is among the nastiest of the tomato diseases. It is a soil-borne fungus famed for its tree-destroying capacity, but it also feasts on every member of the Solanacea, or Nightshade genus. That means, tomatoes, eggplants, potatoes, green peppers, red peppers, hot peppers–our favorite garden staples.
Verticillium wilt is caused by two fungi, Verticillium albo-atrum and Verticillium dahliae. It is part of a complex of destructive fungi whose close relatives include the pathogens of Fusarium wilt and Damp-off of seeds. What all these have in common, regardless of the exact organism responsible, is contaminated soil and the ability to kill with lightening speed or linger in soil to re-attack sickly survivors the following season..
Sometimes it is a nematode that opens the plant to attack. At other times, it is damaged root-hairs from transplant. Still other times it is the natural result of fast growing young roots pushing through hard soil. But once the fungus has its opening, it moves in for the kill, destroying everything it touches and secreting poisons that amplify the damage into areas where the fungus itself has yet to spread. As it invades its new host, the plant’s immune system responds by trying to isolate the killer and contain the damage.
The vascular systems that transport water and nutrients throughout will fill with gum and dead tissue as the plant surgically isolates the organism. The first sign of trouble is a few curling leaves (perhaps at the very top of the plant), stunted growth, a mysterious failure to thrive. But as the damage becomes systemic, the “pipes” that run the length of the stem, xylem and phloem, will fill with gum and become brown and non-functional. Once cross-sectioned, these non-working tissues will appear as brown circles. If the plant is sectioned lengthwise, the dead wood will be seen running its length. The veins in the leaves will brown or display odd colors like yellow or red as the leaf slowly dies. Your plant faces the conundrum: “Water, water, everywhere, but ne’er a drop to drink.”
Unfortunately this is a particularly difficult disease to control. Common tactics such as applying fungicides, collecting and burning infected material and improving drainage are not enough to tame this demon. This is a hardy pathogen that has the ability, in common with major pathogens of humans and animals such as anthrax, botulism, and tetanus, to go dormant and lurk in infected soil for up to seven years.
It is a real nasty customer, found as readily in virgin soil reclaimed from forest, bramble and thicket as in extensively planted ground. And it is promiscuous in its attack, laying waste to strawberries, chrysanthemums and several families of trees and shrubs.
So what remains? Try a modern tomato hybrid with genes for resistance. Seed catalogs are filled with possibilities. These are the routine choice of commercial tomato growers. The tomato grower is lucky here. This is not an option for many other Solanacea growers.
The second possibility is a long crop rotation. The infected soil will be unavailable for growing tomatoes, all other related Solanacea plants and a host of susceptible trees and herbaceous shrubs for many, many years. It will be close to a decade before the susceptible plants can be grown there So you may wish to rotate in plants that do not host the pest, perhaps resistant trees such as pears, apples or walnuts. Cereals or grasses also work well.
Unfortunately the gardener is not in for a skirmish, but a long, long war.