April 2008
Early identification can prevent early die in following years

Verticillium wilt, a major component of potato early dying, is caused by the soilborne fungus, Verticillium dahliae, which causes wilt and crop loss in a range of plants and produces microsclerotia in infected plant tissues. Microsclerotia have thick cell walls that are resistant to harsh conditions, allowing the fungus to live in the soil for up to 15 years without a suitable host plant.

Infection by V. dahliae occurs at the root tip then travels up into the vascular tissue of the plant where it produces a toxin that disrupts the vascular system and prevents water from moving out to the leaves. Symptoms of infection include stunted plants, chlorosis, necrosis and wilt. There are a number of resistant potato varieties, including the moderately resistant Ranger and Umatilla russets. Russet Burbank is moderately susceptible, with stunting, chlorosis and wilt noticeable 13 weeks after inoculation. Russet Norkotah is very susceptible, with necrosis and wilt evident after four and a half weeks and plant death occurring after seven weeks.

Scouting and disease diagnosis can prevent yield loss in subsequent years because verticillium wilt can look like other disorders, but the treatment won’t have an effect. A few weeks after inoculation the symptoms look like a nitrogen deficiency, but applying N won’t do any good. One way to tell if a plant has verticillium is to check for unilateral wilt, where one side has wilt and the other is healthy. The disease also will occur in clumps the first year and if preventive measures aren’t taken it will be widespread by years two and three because plant material breaks down and spreads the fungus.

The first year verticillium wilt appears in a field there will be about 10 percent early dying. The second year it jumps to 40 percent, 60 percent by year three and 80 percent early dying by year four.

Without fumigation and without control measures, verticillium would be a limiting factor for us,” said Dennis Johnson, disease pathologist at Washington State University.

Johnson tested whether infected seed tubers or infested soil had a greater effect on disease incidence. He found that about one in 30 plants from infested seed tubers had problems, but 30 of 30 plants from infested soil had problems.

“Infested soil will give you a severe outbreak every time,” Johnson said. “Clean ground is very important.”

He recommends buying seed that is certified to contain no verticillium because even if the soil already is infested, introduction of a new compatibility group or species of verticillium is possible with infected seed tubers.

A survey of 224 seed lots in 18 states found that 29 percent had verticillium infestations, Johnson said. Almost two thirds of the infested lots had the strain that is most aggressive on potatoes and mint plants, vegetable compatibility group 4A. One third of t he infested lots had VCG 4B, which is mildly agressive on potatoes. The VCG 4A strain of V. dahliae also interacts with the root lesion nematode and causes earlier and more severe symptoms.

Controlling verticillium is difficult once it’s in a field, its long dormancy period means it will stay in the field, so prevention measures are necessary. The main way of controlling the disease is by planting moderately resistant varieties, such as Ranger and Umatilla russets.

“The resistant ones you’re planting restrict the development of the disease and reduce microsclerotia production in infected plant tissue, whus giving yourself an advantage further down the line,” Johnson said.

Some weeds can host verticillium, including pigsweed, nightshade and lambsquarter. controlling host plants will prevent microsclerotia from building up in the soil and preventing further crop loss.

Infested soil can easily be spread to other fields on equipment, so Johnson recommends hosing down tractors, harvesters and planters in the infested field before moving on to another one. Knocking off loose soil should be sufficient to prevent the spread of microsclerotia.

It’s also important to maintain the plant nutrition, Johnson said. Maintaining an adequate soil moisture level can help control the disease as well and is fully explained, as well as all aspoects of managing the disease, in “Potato Health Managememen,” second edition, available at www.shopappress.org.

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