Foliar Diseases of Potato

In Idaho two diseases are most common on potato foliage. These are early blight and white mold. Late blight can occur in Idaho but it is appearance has been extremely sporadic in recent years. Below is a list of some common foliar diseases found in Idaho. 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.

Figure 5. Early blight lesions are characterized by an alternating series of dark concentric rings, surrounded by a narrow band of chlorotic tissue. Figure 5B. Late in the growing season the upper leaves of infected potato plants may be peppered with small early blight lesions.

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.
foliar early blightearly blight 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.

Figure 3a.  As infected tissue decays, hard resting structures called sclerotia (S) form inside the decaying tissue. Figure 3b.  As infected tissue decays, hard resting structures called sclerotia (S) form outside the decaying tissue.

Botrytis Grey Mold

Grey Mold is caused by the fungus Botrytis cinerea . 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.

Botrytis grey mold. Grey mold

Black leg and aerial stem rot

Both of these diseases are caused by pathovars of the bacterium Pectobacterium carotovora. Other bacteria have also be found to be involved in soft rot. The introduction of bacteria is always through a wound in the plant tissue. Black leg (a) is a rot of the lower stem region. This is encouraged by cool, damp conditions. The bacteria may migrate through the soil in water and can reside in plant residue for short periods. The primary inoculum is infected potato seed tubers. Under optimal conditions the bacteria multiply rapidly in the tuber and then spread up the xylem of the shoot killing it. The bacteria will also dissolve the cell walls and liquefy the tuber innards causing soft rot of the tuber. No distinct smell is present in true soft rot. Control is achieved by planting clean seed and rotating the crop. Aerial stem rot (b) is caused by infection through wounds in the stem. Aerial stem rot is often found after plants have been damaged by hail. Aerial stem rot can be differentiated from black leg because aerial stem rot occurs higher up the stem and spreads down towards the ground, whereas black leg occurs from the ground up.

Black leg caused by infection of seed tubers by Pectobacterium atrosepticum Aerial stem rot caused by Pectobacterium carotovora