Field Guide     Crop Diseases    Alternaria Leaf Spot

Alternaria Leaf Spot

CROPS IMPACTED: Tomato, cabbage, carrots, crucifers, beans, potato, onion, pepper, cucumber, and other herbaceous plants

Alternaria Leaf Spot

Family: Pleosporaceae

Alternaria Leaf Spot  Alternaria Leaf Spot

About Alternaria Leaf Spot

Life Cycle

This pathogen is a saprobe, meaning it survives on dead plant material. It can survive off of plant debris both on the surface of soil or below ground. It can also survive in plant seeds. In the spring the fungi will produce spores that are spread by the wind or rain. Assuming the weather conditions are viable, secondary cycles can occur. Once the pathogen has infected its host, leaves will develop an array of dark spots. Eventually as the plant weakens, they will fall and survive on plant debris for next season.

Alternaria Leaf Spot Identification and Habitat


An infected host will develop a number of lesions on the surface of leaves. Lesions will appear as small circular spots with light brown centers surrounded by dark concentric rings. Spotting is most prevalent on lower, older leaves, but new leaves are susceptible too. Spots that are close together may combine to form one large spot. Heavily spotted leaves will quickly turn yellow, wilt, and fall due to lack of water. Leaves suffer the most from these symptoms, but stems, flowers, and seed pods can all be affected.


Warm, moist weather promotes the development of alternaria leaf spot. Wet and moist weather are important in order for spores to germinate properly. Fungal development and dispersal is at its peak with temperatures of 70 to 90°F and about 8 hours of wetness on leaves. Mid-season plants are less vulnerable to disease compared to younger and older plants. Plants lacking proper nutrition and/or growing conditions will be more susceptible to disease.

Alternaria Leaf Spot Management and Control Methods

Cultural Control

Primarily, removing diseased plants upon detection and at the end of the season will deter further development and spread. The spores of the disease rest in dead plant materials, so this is an important step in reducing fungi populations. Proper seed selection is a necessary preventative measure to avoid introducing new diseases to new areas. Providing wider space between plants will help air circulation and sunlight penetration, allowing plants to dry faster. The longer plants stay wet, the more favorable it is for fungal development. Also, if there are any tall grasses or weeds surrounding crops, removing them can help lower humidity levels and promote air flow. Tilling in the fall will aid in breaking down the remaining residue left from infected plants. Rotating crops for one or two years can be useful if there’s an economically viable alternative crop.

Chemical Control

Fungicide is the most viable chemical control for controlling alternaria leaf spot. As long as applications are properly timed, disease should be less incidental after application. Treating seeds with fungicide can also help reduce chances of infection. When using fungicides, be sure to read the label for cautionary advice and application guidelines.

Alternative Alternaria Leaf Spot Names

  • • Early Blight
  • • Purple Blotch
  • • Target Leaf Spot
  • • Alternaria Leaf Blight