Potato Early Blight

Early blight is a common fungal disease of solanaceous crops (tomatoes, potatoes, peppers, eggplants) caused by Alternaria solani. Symptoms first appear as circular dark-brown spots on leaves and stems that can later develop concentric, target-like rings, often surrounded by yellow margins. Lesions are sometimes limited by veins, giving an ‘angular’ appearance. Early foliar symptoms often appear near the base of the plant, spreading up to higher leaves as the disease progresses. On tubers, lesions appear as dark, sunken, cork-like spots with raised margins, although tuber symptoms are less frequently seen in the Midwestern U.S.

Infection & Favorable Conditions

When conditions are right, overwintering spores (conidia) can travel within and between fields and infect healthy plant tissue. Spores can spread via wind, splashing rain, irrigation water, equipment, and workers. Spores can then spread locally from infected tissue in a similar manner. Favorable conditions for early blight development and spread include canopy closure, high humidity, prolonged leaf wetness, poor airflow, nitrogen deficiency. Older or senescing leaves are most susceptible to early blight infection. Once the growing season is over, the pathogen can overwinter on infected soil-bound plant debris for several years.

Disease Cycle

At the end of the growing season, A. solani spores (conidia) and fungal threads (mycelium) are able to overwinter on diseased plant debris in the soil. Once temperature and wetness increase in the spring, conidia are primarily spread via wind or splashing water. These conidia then infect healthy plant tissue through the foliar plant parts including the leaf and stem surfaces, stomata, or wounds, causing lesions to form. These lesions will develop conidiophores, which produce airborne and waterborne spores. These spores will then spread to cause secondary infections on the leaves. Alternaria solani conidia present on soil-bound plant debris can also infect tubers wounded during harvest, although this is not commonly seen in the Midwest. At the end of the cropping season, the pathogen will be present in the field as spores (conidia) or mycelium in the soil, continuing the disease cycle.

Cultural Control

Scouting regularly allows early identification of disease before significant spread and damage. The following practices can also help prevent disease development:

  • Plant resistant (possibly late season) varieties when possible
  • Rotate away from susceptible solanaceous crops (3-4 years)
  • Maximize distance between susceptible fields
  • Maintain proper plant nutrition with appropriate nitrogen fertilization
  • Avoid over-irrigating (reduce leaf wetness)
  • Maintain proper spacing between plants (promote airflow)
  • Destroy or deep-plow infested plant debris
  • Monitor potato physiological days (P-Days) with the VDIFN disease modeling tool described above

Chemical Control

Fungicides can provide good control of early blight in vegetables when applied early on in infection. Multiple applications of fungicide are often necessary to sustain disease management to time of harvest due to the typically high abundance of inoculum and susceptibility of most common cultivars. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website which is annually updated. 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.

Predictive Modeling

Early blight can be modeled using what are called Potato Physiological Days (P-days), which were originally developed to model the progress of potato plants through the growing season. P-days were later implemented to quantify the amount of heat energy to promote early blight disease development throughout a season. Conceptually, they are similar to Growing Degree Days, which are used to track how plants and insects develop over a season in response to daily air temperature highs and lows.

To view the predicted early blight risk on any given day, visit the Vegetable Disease and Insect Forecasting Network (VDIFN) website. From the Disease tab, select the “Early Blight” model. This model uses P-days computed from gridded NOAA weather data to calculate the risk of early blight development, which is displayed as a colored map overlay. The start point should be set to the date of approximately 50% crop emergence for potato. For tomato, pepper, or eggplant transplants, the start point should be the date of transplanting to the field.

At the accumulation of 300 P-days, after crop emergence in a season at a given location, temperature conditions have been met to support early blight infection on susceptible host crops. This provides a time at which crops should be intensively scouted for first infection and/or protected with preventative fungicide treatments to reduce first infection. This initial threshold is critical because first infection creates lesions and news spores which establish the overall in-field inoculum load for the rest of the production season. In concentrated vegetable production regions, A. solani inoculum is typically highly abundant. Beyond the risk threshold of P-day 300, daily risk scores are calculated from the average daily P-day accumulation over the past week. High accumulations indicate enhanced early blight disease risk. Click any grid point in VDIFN to get more detailed weather and disease progression information for that location.


  • Our page on Weather and Potato Disease Models. Access weather data charts and potato disease risk models for early blight and late blight for multiple locations across Wisconsin.
  • Vegetable Disease and Insect Forecasting Network (VDIFN). Interactive map showing estimated late blight risk based on disease severity value (DSV) accumulation since crop emergence or date of last fungicide application.
  • 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 Nov 2023

Early blight foliar lesions on infected potato plant. Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Close-up view of early blight lesion with concentric rings on potato leaf. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org