Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu
In recent years, Alternaria leaf spot and head rot has been damaging to fall cole crops, especially broccoli, in Delaware. Broccoli is the crop most susceptible to Alternaria disease, followed by collards. Kale, cauliflower and cabbage can be affected by Alternaria but are less susceptible than broccoli and collards. Alternaria causes circular leaf spots with concentric rings. Uncontrolled foliar symptoms can spread disease to broccoli, cabbage and cauliflower heads, rendering them unmarketable.
Alternaria leaf spot on broccoli.
Alternaria is favored by the cool wet conditions often present in the fall. Temperatures of 55-75 °F, rain or heavy dew allow the disease to develop and spread. Alternaria spores can be spread by wind, rain splash or flea beetles.
Alternaria overwinters in crop residue. It can also persist on brassica family weeds (i.e. mustard) and common non-brassica weeds like evening primrose and lambsquarters. The disease can also enter a field through infested seed. A recent study found that more than 30% of commercial seed lots tested were infested with Alternaria brassicicola. Some of the Alternaria isolates recovered from seed had reduced sensitivity to azoxystrobin fungicide (Quadris).
Alternaria Management Recommendations
Reduce Risk of Disease Presence
To reduce the risk of Alternaria entering a cole crop planting, rotate fields away from brassica crops for at least three years. Manage weeds in and around fields. To reduce risk from infested seed, use hot water seed treatment. Manage flea beetles, which can spread Alternaria over distances from other cole crops or weed hosts.
Manage the Field Environment
Dense plant spacing, overhead irrigation and succession planting of brassica crops all increase risk of Alternaria spread. If Alternaria has been present on your farm in past years, consider using wider plant spacing, drip irrigating and spatially separating earlier and later plantings.
Choose Tolerant Varieties
In inoculated variety trials in Georgia, the following broccoli varieties had lower Alternaria disease severity and higher marketable yields: Vallejo, Emerald Pride, Burney, Belstar, Ironman, and Marathon. The most susceptible varieties in the Georgia trials were Eastern Crown, Emerald Jewel, Lieutenant, and Abrams.
Use Recommended Fungicides
Consult the Mid-Atlantic Commercial Vegetable Production Recommendations for recommended fungicides. Initiate fungicides applications before disease symptoms appear and repeat every 7-10 days if conditions favor disease development. Resistance to FRAC code 7 and 11 fungicides have been reported in other states. If you are applying fungicides and not seeing effective control, rotate to a different FRAC code fungicide.
For organic production, Oso and Kocide have suppressed Alternaria and increased marketable yield in trials done by Cornell University.
References
Cassity-Duffey, K., Coolong, T., Dutta, B. & McAvoy, T. Assessing broccoli cultivar susceptibility to Alternaria brassicicola in Georgia. Plant Health Progress 25, 484–491 (2024).
Kaur, N. & Dutta, B. Aggressive Alternaria brassicicola with reduced fungicide sensitivity can be associated with naturally infested broccoli seeds. Plant Disease 108, 2154–2161 (2024).
Highlights from “Control-Alt-Delete” Alternaria Leaf Spot and Head Rot in Broccoli Project
Susan Scheufele