Alternaria Leaf Blight

Alternaria leaf blight, also known as black rot or Alternaria leaf spot, is a conglomerate of leaf spot diseases.

Alternaria fungi are one of the most destructive plant pathogens, causing an estimated 20% of crop losses worldwide. Those numbers can go as high as 80% in location-specific infections. Making matters worse, many people are allergic to Alternaria fungi, a type of black mold. Exposure can trigger asthma attacks and other respiratory problems. But what about your plants?

Melon leaf infected with Alternaria leaf blight (Howard F. Schwartz, CSU, Bugwood.org) CC BY 3.0 US

This fungal disease collective caused by various types of Alternaria fungi. Depending on which form of Alternaria is present, almonds, apples, plants in the cabbage family, calendula and marigolds, carrots, cherries, citrus, escarole, mango, papaya, parsley, potatoes, squash family, and tomatoes are all susceptible to one type of Alternaria leaf spot or another.

Alternaria leaf blight symptoms

Symptoms of Alternaria leaf blight tend to be family-specific. Generally speaking, brown smudges first appear on older leaves, near the base or crown, of the plant. If you look closely, you may be able to see yellow halos around these damaged areas. You may also see concentric, target-shaped rings that darken as the disease progresses or a shot-hole effect similar to early blight. Ultimately, leaves wither, curl upward and die.

Tagetes marigold infected with Alternaria leaf spot (Scot Nelson) Public Domain

When broccoli, cauliflower, and other members of the cabbage family are infected with Alternaria leaf blight, the heads deteriorate rapidly and look unappetizing. Infected cherries, papayas, and tomatoes will exhibit dark, round, flattened lesions on the fruit. Similar in appearance to blossom end rot, Alternaria leaf spot damage can appear anywhere on infected fruit.

While Alternaria leaf blight doesn’t always attack your crops directly, it does interfere with photosynthesis enough to reduce crop size and can lead to sunburn damage. It also creates points of entry for other pests and diseases.

Alternaria leaf blight on Chinese kale (S. Seemadua) CCY BY 3.0 AU

Alternaria leaf blight control

Nine hours of continuous wetness sets the stage for Alternaria to attack plant leaves. This is especially true when temperatures range from 70°F to 82°F. Thirteen hours later, spores start germinating. Since Alternaria leaf blight spores can travel for over a mile on the wind, it’s important to use good cultural practices to reduce the odds of infection in the first place. These practices include:

• use certified disease-free seeds and seedlings
• space and prune plants to maintain good airflow
• avoid overhead watering
• do not work around plants while they are wet
• employ crop rotation
• apply mulch around but not touching plants
• dispose of infected plant material in the trash

There are fungicides rated for use against Alternaria leaf blight, but I’d rather prevent the problem in the first place.