Southern blight of apple new disease for Pennsylvania fruit growers

The 2021 growing season has been producing a couple of surprises, the latest being increased incidences of southern blight on apple.

Southern blight of apple was first observed in Pennsylvania in 2018 and, unfortunately, is here to stay. The fungus affects the lower part of the tree, killing roots, and girdling the tree. The causal organism has been identified as Sclerotium delphinii (formerly known as Sclerotium rolfsii var. delphinii), which is a soilborne pathogen. Southern blight of apple has been primarily found in apple-growing regions of the southeastern United States where wet, humid weather favors the disease.

Summer conditions during the last several years in Pennsylvania, particularly in the southern region, have felt a lot like conditions in the southeast, so it is not surprising this pathogen has become more active further north. The disease is most common on young apple trees (less than 3 years old), but older dwarf trees have been observed to be susceptible, as well, under the right conditions.

Figure 2. Aboveground symptoms of apple trees affected by southern blight. Photo: Kari Peter, Penn State

S. rolfsii is a widespread pathogen affecting several hundred plant species, including soybean, clover, and tomato. Since the disease has a broad host range, chances are the fungus has been present in our soils for a while; however, our conditions did not favor the disease until recently. As a soilborne pathogen, it spreads by the movement of soil and/or diseased plant material. Since this is a soilborne fungus and does not move by air currents, orchards are self-infecting.

To date, this is not widespread in orchard soils throughout Pennsylvania; however, the disease is causing headaches for some folks, especially in Adams County. Growers in Maryland and the southern half of Pennsylvania have experienced the ideal conditions for the disease during the 2021 season and folks should be on guard. It is important growers be alert for suddenly declining trees, especially young, newly planted trees. Investigate the trunk close to the soil line for signs of the fungus. If the tree is positive for infection, the tree should be dug up carefully, along with the surrounding soil, and removed from the orchard, to limit the spread and perpetuation of the fungus within the orchard.

What to look for in an infected tree

Regardless of host, the pathogen produces similar disease signs and symptoms. During hot, humid conditions, sclerotia (the fruiting body of the fungus) germinate producing a web of white mycelium that can infect the apple tree. The signs of the fungus are 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 thread-like, 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.

The sclerotia may also be found at the base of the tree and on roots up to 5 inches deep in the soil. There are often white mycelia throughout the soil of the roots of an infected tree. When the fungus attacks the roots, the leaves will wilt and show characteristic yellowing or reddening discoloration indicative of the crown being girdled.

Figure 3. (left) 13 yr old dwarf apple tree showing signs of southern blight with many sclerotia at the base of the trunk at the soil line. (right)Signs of the fungus causing southern blight on apple: Roots exposed from an infected tree with white mycelium throughout the soil. Photos: Kari Peter, Penn State

How the fungus survives and causes disease

Sclerotia are the principal means of overwintering and long-term survival of the fungus. Dispersal by air movement is not significant since the fungus rarely produces spores. Infection usually takes place directly by the penetration of young, uninjured bark and roots, but injuries to the bark and roots may facilitate entry of the pathogen. Factors that promote a high incidence of disease are summer temperatures 77–95°F (25–35°C), high levels of soil moisture, good aeration, and an abundance of organic debris. The distribution of disease in the orchards rarely shows a pattern. Young trees are most susceptible; however, under ideal conditions, older dwarf trees are vulnerable.

Figure 4. Signs of the fungus causing southern blight: White mycelia with sclerotia attached on the lower part of the trunk of a 4 yr old apple tree. Photo: Kari Peter, Penn State

How to manage the disease

Unfortunately, there are no chemicals labeled to manage southern blight on apple. The best management strategies to date are through cultural controls, such as controlling weeds around the tree collar, rouging diseased trees, and avoiding fields known to be infested or planted previously to soybean, tomato, or clover. A similar pathogen causes the disease in these crops. The pathogen can be controlled by fumigation. All apple rootstocks are susceptible.

– Kari A. Peter, Penn State University