Apple Disease - Southern Blight
Southern blight of apple has been primarily found in the southern half of the United States (North Carolina and South Carolina to California). It is most common on young trees (less than 3 years old), but older dwarf trees have been observed to be susceptible, as well, under the right conditions.
Symptoms
A web of white mycelium is often found on the soil and/or the trunk at the base of an infected tree. Under moist conditions, the mycelium may progress up the trunk several inches in a fanlike pattern. Within a few days it disappears, and during periods of high humidity, masses of sclerotia develop. The sclerotia are white to tan at first and tan brown at maturity. They may also be found at the base of the tree and on roots up to 5 inches deep in the soil. There is often white mycelia throughout the soil of the roots of an infected tree. When the fungus attacks, the leaves will wilt and show characteristic yellowing or reddening discoloration indicative of the crown being girdled.
Disease cycle
Sclerotia are the principal means of overwintering and long-term survival of the fungus. Infection usually takes place directly by the penetration of young, uninjured bark, but injuries to the bark may facilitate entry of the pathogen. Factors that promote high incidence of disease are summer temperatures 77 - 95°F, high levels of soil moisture, good aeration, and an abundance of organic debris. The distribution of disease in the field rarely shows a pattern. Young trees are most susceptible; however, under ideal conditions, older dwarf trees are vulnerable.
Disease management
Cultural control methods include controlling weeds around the tree collar, rouging diseased trees, and avoiding fields known to be infested or planted previously to soybean. A similar pathogen causes a disease in soybean. The pathogen can be controlled by fumigation. To date, there are currently no fungicides labeled for soil applications to apple to prevent southern blight. Research is ongoing. All apple rootstocks are susceptible.
