Foliar Diseases/Conditions of Potato

In Michigan two diseases are most common on potato foliage. These are late blight and early blight. Below is a list of some common foliar diseases that may be confused with late blight. Many of these organisms also can infect tubers. For information on tuber diseases please visit the tuber diseases page.

Late Blight of Potato
Late blight is caused by the fungus-like oomycete Phytophthora infestans. This organism is generally treated as a fungus and can cause both foliar and tuber infections. The pathogen is dispersed via wind-borne sporangia which produce flagellated zoospores. These zoospores can infect leaf tissue through either direct penetration of the epidermis or through stomata. Once inside the plant the pathogen begins to kill the tissue, causing the distinct foliar lesions. Control is accomplished by using a protectant fungicide program.

Early Blight of Potato
Early blight is caused by the fungus Alternaria solani. Conidia are approximately 200 µm long and have several transverse septa. The conidia possess a tail or beak. The fungus overwinters in debris and is hardy in Michigan. The lesions produced are often confused with late blight. Early blight lesions show a general dry "bulls-eye" pattern and do not usually spread very far and rarely affect petiole tissue. In contrast, late blight lesions are appear watery and spread down the petiole and the stem. A. solani can cause tuber rot but unlike late blight, infected tubers are not generally susceptible to colonization of secondary pathogens. Protectant fungicide programs used to control late blight are generally effective at controlling early blight. A related fungus Alternaria alternata produces similar lesions.

White Mold of Potato
White mold, also called Sclerotinia stem rot, is caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary. It is prevalent in the Pacific Northwest but in Michigan it is of secondary importance except in wet seasons or under excessive irrigation. The disease favors very moist conditions and is especially common in fields with overhead-irrigation such as by means of a center pivot. Agricultural practices that promote extensive canopy growth and keep relative humidity and free moisture in the crop canopy for extended periods of time and reduce wind movement, favor disease development.

Botrytis Leaf Rot of Potatoes
Botrytis leaf rot is caused by the fungus Botrytis sp. This pathogen also causes gray mold. The disease is usually the result of excessive humidity and other stress and does not usually pose a large threat to the crop. The fungus overwinters on plant debris. Tuber infection can occur if the inoculum levels are high and the storage facility is very humidity. Standard fungicide protection programs offer control when conditions are not too wet.

Leaf Roll Virus
This virus is aphid vectored and tuber borne. Foliar symptoms include upward rolling of the leaves and mild chlorosis. The virus damages phloem tissue and can infect tuber tissue. The only control mechanisms are to plant virus free seed and to control aphid populations in the field.

Mosaic Virus
This virus causes leaf mottling and a reduction is plant size. Infections can reduce tuber yield and quality. The virus is vectored in a similar manner to that of leaf roll and can be controlled by using virus free seed and controlling aphid populations.

Verticillium Wilt Foliar Symptoms
The disease is caused by fungi in the genus Verticillium. The fungus causes foliar wilting, chlorosis, and discoloration of the vascular ring in both the stem and tuber. Tuber infections may be confused with late blight. Verticillium overwinters in the soil and be can be control by using clean seed, seed treatments, and rotations to reduce soil inoculum levels.