Brown spot and Black pit are fungal diseases of potato caused by the fungus Alternaria alternata. On leaves, it causes relatively small dark brown spots of necrotic tissue with a dark brown margin. Starting as small lesions, the spots can coalesce to cover a large percentage of leaf or petiole surface. On tubers, the disease causes black, deep sunken pits with definite margins that often develop in storage. The pathogen causing these diseases survives on infected soil-bound plant debris and susceptible weeds. Alternaria alternata is easily confused with A. solani, causal agent of the more common and destructive disease early blight, which affects both potato and tomato plants. Brown spot tends to be favored by warmer temperatures than early blight.
Primary Source: Soil-bound plant debris, susceptible weeds
Spread: Spreads to leaves via wind, and wounded tubers through infected soil
Favorable Conditions: Long dew periods, standing water on foliage, temperatures over 64ºF, reduced airflow, plant maturity, and low nitrogen status
Infection & Disease Cycle
In Wisconsin, A. alternata overwinters on host weeds and in plant debris as spores (conidia) and fungal threads (hyphae). Spores will spread from these sources after heavy rainfalls or changes in relative humidity, with the resulting windborne conidia then coming into contact with potato leaves. Resulting infections are worsened by increased leaf wetness (rainfall, dew, irrigation, etc.) and warmer temperatures. During moist conditions, mature lesions will produce new spores that are able to travel to new plant material. Stressed plants, or plants infected with other pathogens, can be more susceptible to brown spot. Alternaria alternata is a relatively weaker pathogen than A. solani (causing early blight) and can be saprophytic in its activity. Tubers that were both bruised and introduced to soil-bound inoculum during harvest can also become infected.
Scouting regularly allows early identification of disease before significant spread and damage. The following practices can also help prevent disease development:
- Plant certified disease-free seed
- Manage host weeds
- Maintain proper plant nutrition to avoid plant stress
- Rotate away from solanaceous crops for 3 or more years
- Avoid excessive and overhead irrigation
- Destroy or deep-plow infested plant debris
- Avoid bruising tubers during harvest
- Manage other diseases to reduce susceptibility to brown spot
Well-timed foliar protectant, broad spectrum foliar fungicide applications can prevent the development and spread of A. alternata at the first sign of disease or after flowering. The pathogen has shown significant resistance to strobulurins, which should be avoided. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.
- Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
- UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.
Written by Amanda Gevens, Ariana Abbrescia, Russell Groves, and Ben Bradford. Last updated Aug 2023