By Blake Jackson
Southern blight, a destructive fungal disease, is gaining a foothold in Pennsylvania apple orchards. Previously confined to warmer regions, milder winters and occasional wet summers have allowed this pathogen to establish itself further north. First detected in Pennsylvania in 2018, southern blight primarily targets young apple trees, posing a significant threat to orchard production.
The culprit behind this disease is Sclerotium rolfsii, a soilborne fungus with a wide host range. While the fungus itself might have been present for some time, recent weather patterns have created favorable conditions for its spread. Unlike airborne diseases, southern blight moves through contaminated soil and infected plant material, making orchards susceptible to self-infection.
Identifying infected apple trees requires vigilance. During hot and humid periods, look for white, web-like fungal growth (mycelium) at the base of the trunk or on the surrounding soil. This growth may extend several inches up the trunk before disappearing. Later, when humidity rises, brown sclerotia, the fungus's seed-like survival structures, will develop at the base of the tree and around the roots.
These sclerotia allow the fungus to survive winter and re-emerge in favorable conditions. Infection typically occurs through direct penetration of healthy bark and roots, but injuries can also create entry points. Warm summer temperatures, high soil moisture, and abundant organic matter all contribute to the spread of southern blight.
Unfortunately, there are currently no preventative chemical controls for southern blight in apples. However, some cultural practices can help mitigate the risk. Orchard management should focus on controlling weeds around the tree base, removing diseased trees promptly, and avoiding planting in fields previously used for crops like soybeans, tomatoes, or clover, which are known hosts for this fungus.
Sudden decline, especially in young trees, warrants closer inspection of the trunk near the soil line for signs of fungal growth. If southern blight is confirmed, carefully remove the entire tree and surrounding soil to prevent further spread.
Research efforts at Penn State's Tree Fruit Pathology Lab are underway to address this emerging threat. Identifying potential fungicides and exploring resistant apple rootstock varieties are key areas of focus in the fight against southern blight. As the climate continues to change, vigilance and proactive management strategies will be crucial for protecting Pennsylvania's apple orchards from this destructive disease.
Photo Credit: istock-rixipix
Categories: Pennsylvania, Crops, Fruits and Vegetables