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University of Idaho faculty are continually investigating potato related diseases and effective disease management techniques. Our goal is to provide Idaho potato growers with reliable and up-to-date information on the identity, biology, disease-cycles and current control methods for potato diseases.

Foliar Diseases

In Idaho, there are two diseases that are most common on potato foliage: early blight and white mold. Late blight can also occur in Idaho, but its appearance has been extremely sporadic in recent years. Below is a list of common potato foliar diseases found in Idaho.

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 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 is caused by infection through wounds in the stem and is often found after plants have been damaged by hail. Aerial stem rot can be differentiated from black leg because it occurs higher up the stem and spreads down towards the ground, whereas black leg occurs from the ground up.

Gray Mold is caused by the fungus Botrytis cinerea. 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 humid. Standard fungicide protection programs offer control when conditions are not too wet.

Early blight is caused by the fungus Alternaria solani. The disease primarily affects leaves and stems, but under favorable weather conditions, and if left uncontrolled, can result in considerable defoliation and enhance the chance for tuber infection. Premature defoliation may lead to considerable reduction in yield. Protectant fungicide programs used to control late blight are generally effective at controlling early blight. Learn more...

Late blight is caused by the fungus-like oomycete Phytophthora infestans. It can infect and destroy the leaves, stems, fruits, and tubers of potato and tomato plants. 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 issue 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. Learn more...

White mold, also called Sclerotinia stem rot, is caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary (Discomycetes, Helotiales). White mold is favored by moist conditions and commonly occurs in fields with overhead irrigation, especially under a center pivot. Disease development is enhanced by agricultural practices that promote extensive canopy growth, keep humidity and free moisture in the crop canopy for extended periods of time and reduce wind movement. Learn more...

Tuber Diseases

There are a number of fungi, fungal-like organisms and bacteria capable of causing disease in potato tubers. Below is a list of common potato tuber diseases.

Black dot is caused by Colletotrichum coccades (Wallr.) Hughes., and is a common disease of potato. It is most often observed on tubers but it can affect all parts of the plant. The symptoms of black dot are similar to more common potato diseases. On potato foliage, symptoms are nearly indistinguishable from early blight and on tubers it produces blemishes that are easily mistaken for silver scurf. Black dot affects all parts of the plant. Learn more...

Seed-borne diseases represent a significant constraint to potato production in the US. Pathogens such as Phytophthora infestans (late blight) and Fusarium sambucinum (Fusarium dry rot) affect tubers in storage and seed tubers and sprouts after planting. In severe outbreaks the pathogens may kill developing sprouts outright resulting in delayed or non-emergence. Read more...

Common scab may be caused by several soil dwelling plant pathogenic bacterial species in the genus Streptomyces, including S. scabies and S. turgidiscabies. The disease occurs worldwide wherever potatoes are grown. Although scab does not usually affect total yields, since the marketplace for potatoes is quality driven, the presence of scab lesions, especially those which are pitted, significantly lessens the marketability for both table stock and processing varieties. Learn more...

Fusarium dry rot is one of the most important diseases of potato, affecting tubers in storage and seed pieces after planting. Fusarium dry rot of seed tubers can reduce crop establishment by killing developing potato sprouts. Crop losses can be up to 25%, while more than 60% of tubers can be infected in storage. All the commonly grown potato cultivars in North America are susceptible to the pathogen, although some are less susceptible than others and several breeding lines have been reported to have a higher degree of resistance to dry rot. Learn more...

Both diseases are caused by Rhizoctonia solani. It rarely produces spores (residing in the soil as sclerotia) and attacks a variety of plant species. 

Rhizoctonia canker occurs when stolons contact soil-borne fungal bodies. The fungus infects plant tissue and causes stolon blinding, reducing tuber production and yield. 

It also infects tubers causing black scurf. This is purely cosmetic and does not reduce yield. Control is achieved by seed treatments, rotations with non-susceptible crops, clean seed, and possibly soil fumigation.

Learn more...

Pink Rot is caused by Phytophthora erythroseptica, a cousin of the late blight pathogen. The pathogen does not infect foliar tissue. Foliar symptoms of underground infections include wilting and chlorosis. Tubers become infected through diseased stolons and show darkened diseased area on the skin. The rotted tissues remain firm and become slightly spongy. If the tuber is cut the tissue oxidizes to a pinkish tinge, an easy diagnostic characteristic. Learn more...

Late blight infection of tubers is characterized by irregularly shaped, slightly depressed brown to purplish areas on the skin. These symptoms may be less obvious on russet and red-skinned cultivars. A tan to reddish-brown, dry, granular rot is found under the skin in the discolored area, extending into the tuber usually less than 1/2 inch. Late blight rot of tubers is often accompanied by soft rot, and in many cases more than one fungal pathogen may infect tubers at the same time.

Pythium Leak is a water mold fungus that is in the same family as Phytophthora infestans. Unlike P. infestans, Pythium does not infect foliar tissue. This genus of pathogens attack a wide range of plants and cause diseases like crown rots and damping off. Infection occurs when the fungus enters the tuber through a wound. The rot is encouraged by hot weather and can lead to serious losses. Tubers show a general darkening of the tissue and the presence of liquid exudates when compressed. Eventually the pathogen degrades all but the outer shell of the tuber. Losses can be minimized by harvesting in cool weather and allowing the potato skin to develop before harvesting.

Silver Scurf is caused by Helminthosporium solani. Spores are soil borne and cause brown lesions on the potato surface. These lesions are primarily cosmetic. Symptoms develop under moist conditions and often are not observed until the tubers have been stored for a period of time. Control can be achieved by planting clean seed, harvesting the tubers when mature, and storing tubers under cool conditions without excessive moisture.

Black Heart is not a disease of any sort, but a physiological condition. Black heart occurs primarily in storage when the tubers do not receive enough oxygen. The tissue dies from the inside out and turns jet black. A smell is absent. Increasing air flow and decreasing storage density can halt this condition. The tubers will not recover once Black Heart has occurred.

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