Late Blight and Its Lookalikes: Correctly Identify Late Blight in Potatoes

Agriculture
Vegetables

It is important to familiarize yourself with how late blight can appear in crop, and what diseases and conditions may be similar. The descriptions provide information on various potato diseases, and how their symptoms differ from late blight.

Late Blight

Late blight spreads during wet or humid periods, with higher disease incidence when it is cool and wet. Leaf symptoms: light to dark-green water-soaked spots that will turn brown over time, light green halo will form around the lesion, and white spores will form on the underside of leaves. When conditions are hot and dry like we have been experiencing lately, the brown lesions on the leaves can dry up and spores will disappear making it difficult to identify. Leaf lesions will cross over the mid-vein of the leaf if it comes in contact with the lesion. Lesions and white spores will also form on stems, especially at growing points. Attention should be given especially to parts of the fields that exhibit high humidity, such as low-lying areas, along hedgerows, near weedy patches, and near water.

Late Blight Sporulation

The underside of potato leaves showing sporulating lesions.

Photo provided by Ontario Crop IPM

Two round brown spots can be seen on the back of two potato leaves surrounded by a halo of white spores.

Late Blight on the Growing Point

The top growing point of a potato plant is brown and wilting as a result of late blight infection.

Photo provided by Ontario Crop IPM

The top growing point of a potato plant is pictured and is brown and wilting as a result of late blight infection.

Late Blight in Dry Conditions

Under dry conditions, late blight will appear as dry brown patches.

Photo by Ontario Crop IPM

Two potato leaves both have large brown spots from the tip of the leaf up the center. Both leaves are curled as they are dried out.

Lookalikes

Early Blight

Brown circular lesions form on leaves with some yellow halos, and browning on stems. However, lesions form a concentric circle “bullseye” pattern with no white spores, and stem lesions show up as flecks instead of larger infected areas.

Photo by Margie Lund, Cornell Vegetable Program

Grey Mold

Dark lesions and mold spores form on leaves. However, lesions form a concentric circle “bullseye” pattern and mold spores are grey in color compared to the white spores that form with late blight. Lesions will form on the tips of the leaves and across the mid-vein like late blight, so looking for the concentric circle patter is a good indicator.

Photo by Ontario Crop IPM

Alternaria Brown Spot

Brown lesions will form on leaves and stems. However, lesions usually start out very small and older lesions grow larger with a concentric circle “bullseye” pattern. Lesions on stems will be small and scattered spots.

Photo by Ontario Crop IPM

White Mold

White mold also has white spore growth on foliage, but this mold forms denser fungal growths and areas are not brown to black like late blight. Infected areas turn white in color and dark hard sclerotia will form on stems.

Photo by Ontario Crop IPM

Rhizoctonia

White fungal growth also occurs on stems, but this growth can easily be wiped off and stem tissue under the fungus is not damaged.

Photo by Ontario Crop IPM

Blackleg

Blackleg causes blackened rotting stems in potatoes. However, no white fungal growth will develop, and plants will sometimes produce a fishy smell. Rotting stems develop at the soil line and work their way up the plant.

Photo by Margie Lund, Cornell Vegetable Program

Other Conditions

Heat Stress and Tip Burn

Under heat stress, potato leaves will turn brown. Sometimes full leaves will wilt, and other times browning will occur just along the tips of the leaflets. However, no white spores will form, and brown spots will develop during times of extreme heat and dry conditions.

Water Damage

Water-soaked brown spots form with leaf yellowing surrounding the lesions. However, no white spores will form.

Chemical Damage

Growing points and leaves develop dark lesions. However, lesions will often only form in the boundaries of the leaf veins instead of crossing over veins. Additionally, no spores or yellowing around lesions will form.