Tuber and Stem Diseases of Potato

Black Dot

Black dot, caused by Colletotrichum coccodes (Wallr.) Hughes., is a common disease of potato. It is most often observed on tubers but it can affect all parts of the plant. The disease has probably been underestimated in the recent past as the symptoms 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. Although not as serious as other more common potato diseases such as black scurf, silver scurf, or common scab, it can be more devastating as it affects all parts of the plant. Above ground it can infect the vascular system causing wilt, and below ground it can cause severe rotting of roots, shoots and stolons, leading to early plant decline, discolored tubers and reduced yields.

Black dot microsclerotia. Black dot

Seed Piece Health Management

Seed-borne diseases of potato represent a significant constraint to potato production in the US. Pathogens such as Phytophthora infestans (late blight) and Fusarium sambucinum (Fusarium dry rot), are major pathogens of potato affecting 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. Reduction in crop vigor then results from expenditure of seed energy used to produce secondary or tertiary sprouts to compensate for damage to primary sprouts. Thus, the use of an effective seed treatment in combination with good management practices during cutting and seed storage prior to planting are essential to reducing late blight and Fusarium dry rot, and secondary bacterial soft rot in cut seed prior to planting.

Fusarium dry rot. Tuber late blight

Common Scab

Common scab may be caused by several soil dwelling plant pathogenic bacterial species in the genus Streptomyces, including S. scabies and S. turgidiscabies. In particular, S. scabies has been well documented as causing scab lesions. Streptomyces scabies infects a number of root-grown crops including radish (Raphanus sativus), parsnip (Pastinaca sativa), beet (Beta vulgaris), carrot (Daucus carota), as well as potato (Solanum tuberosum). 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 tablestock and processing varieties.

Figure 1. A common scab infected tuber. Cork-like lesions form on the surface of the tuber. Figure 3. As well as surface and rasied cork-like lesions, common scab may form pitted lesions which may be 3-4 mm deep.

Fusarium Dry Rot

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, and 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.

Figure 1. External symptoms of Fusarium dry rot. Dark depressions form on the surface of the tuber and the skin becomes wrinkled as the underlying tissue dessicates. Figure 2. Internal dry rot symptoms.  Clumps of  mycelium line a dry nectotic cavity hollowed out from rotted tissue.White to pink spore masses form on the tuber surface.

Black Scurf and Rhizoctonia Canker

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 soilborne fungal bodies. The fungus infects plant tissue and causes stolon blinding thus reducing tuber production and yield. It also infects tubers causing black scurf. This is purely cosmetic and does not reduce yield. Control is achieve by seed treatments, rotations with non-susceptible crops, clean seed, and possibly soil fumigation.

Figure 1. Rhizoctonia solani sclerotia on the surface of tubers

Pink Rot

This diseased 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.
internal pink rot symptoms. Photo courtesy of Jeff Miller.  Miller Research Inc.external pink rot symptoms

Tuber Late Blight

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. The extent of rotting in a tuber depends on the susceptibility of the cultivar, temperature and length of time after the initial infection. The margin of diseased tissue is not distinct and is marked by brown finger-like extensions into the healthy tissue of the tuber. In time, the entire tuber becomes blighted and discolored. 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.

Figure 3a.  Late blight infection of tubers is characterized by irregularly shaped, slightly depressed brown to purplish areas on the skin. Figure 3b.  A tan to reddish brown, dry, granular rot is found under the skin in the discolored area.

Pythium Leak

This is 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. They are general soil saprophytes and can survive on soil debris. 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.
pythium rotPythium rot2

Silver Scurf

This disease 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.
Silver scurf

Black Heart

This 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 this has occurred.
Black Heart