Soil born pathogens
Verticillium spp. have a wide host range, including herbaceous annuals and perennials and woody perennials. Verticillium wilt is problematic in temperate areas of the world, especially in irrigated regions. The pathogens can persist in soil for many years in the absence of a susceptible crop. Infection is through the roots, and management of the disease is difficult.
Foliar symptoms first appear as chlorosis and necrosis beginning in the lower leaves. On warm, sunny days, leaves may appear limp and flaccid. Sometimes symptoms occur on only one side of the leaf or the plant
Diseased trees may have sparse foliage, branch dieback, buds which fail to open in the spring, and/or fall coloration that develops a few weeks preceding normal autumn coloration. Infected trees may die; however, sometimes trees can be saved if symptomatic branches are pruned.
Verticillium wilt is difficult to control because it persists in the soil indefinitely. Infected trees that are not yet dead sometimes “outgrow” the fungus. Dead branches should be pruned out to help overall plant vigor. The disease can be transmitted on pruning tools. It is recommended that tools be sterilized by dipping them in a diluted cleanser, such as Lysol, Pinesol, or household bleach, between cuts and between trees.
Management plans for Verticillium wilt invariably include the use of resistant varieties and species, extra separation between plants and use of ammonium nitrogen fertilizer. Adding compost to improve microbial health and soil drainage, limiting irrigation when rain is plentiful, and waiting to plant until soil and air have warmed to near 80 degrees may help limit fungus vitality.
Take measures to prepare your soil before planting. Inoculation of microorganisms and beneficial bacteria and fungi, healthy soil enables plants immune systems to fight off the pathogens.
Steaming soil cooks Verticillium mycelia, but solarizing with a big sheet of black plastic might be the practical choice for an urban garden plot. Steam the soil used for potted plants or for bench crops in the greenhouse and nursery at 180°F (82°C) for 30 minutes or 160°F (71°C) for one hour.
Fertilize to promote vigorous growth and maintain a balance of nitrogen, phosphorus, and potassium. Fertilizing can help reduce symptoms in nursery, field, and landscape plantings. Apply a fertilizer containing ammonium sulfate following the suggestions in a soil test report. Affected trees and shrubs should be fertilized and watered as soon as possible after initial wilt symptoms are exhibited. For quick response, the fertilizer should either be injected into the soil in liquid form or be applied to the soil surface and watered in. Ammonium sulfate can be applied at the rate of 29 pounds per 1,000 square feet. Water well immediately after application. Control weeds that can act as inoculum reservoirs in and around planting sites. Common weed hosts include ground cherries, lamb's-quarter, pigweed, horse nettles, and velvet leaf.
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