Seeing the Black Dot

Crop rotation on the development of Colletotrichum coccodes

Published in the April 2012 Issue Published online: Apr 09, 2012 Dennis Johnson
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Figure 1: Black dot Sclerotia on dead potato stemsTubers, roots, stolons, below- and above-ground stems and leaves of potato are all infected by the soil-borne fungus Colletotrichum coccodes. Infected tissues develop characteristic small, black sclerotia that appear as black dot (Fig. 1), giving the name "black dot" to the disease. Black dot is common in most potato-growing areas in the world and may cause up to 30 percent yield reduction.

Infection of potato by the black dot fungus can be caused by soil-borne, tuber-borne or airborne inoculum. Airborne inoculum is likely when foliage of young plants are wounded by blowing sand. Wounds from blowing sand provide entry avenues for the fungus. Lesions develop on foliage from airborne inoculum and are dark brown to black (Fig. 2) and similar in appearance to early blight, except concentric rings within the lesion do not develop. Lesions on leaves caused by the black dot fungus are generally much less frequent than those caused by the early blight fungi in the Columbia Basin of Washington and Oregon.

The black dot fungus infects below- and above-ground potato stems from soil- and tuber-borne inoculum relatively early in the growing season (Fig. 3). However, infections in stems are usually initially latent in that disease symptoms often do not become evident until late in the growing season. Leaves low in the canopy on infected plants may turn chlorotic and drop after closure between rows and before plant senescence. The fungus rapidly expands within stem tissues during senescence and during times of plant stress; sclerotia generally do not develop until plants begin to senesce and die.

Potato roots and stolons are very susceptible to infection. Lesions on below-ground plant parts may resemble those caused by Rhizoctonia. The cortical tissue is invaded, resulting in a sloughing of the periderm (Fig. 4). As stems dry, cortical tissue is easily scaled away and an amethyst color may be common inside the vascular cylinder. Sclerotia develop in infected tissues. Potato roots are more extensively colonized by the black dot fungus when plants are stressed. In recent experiments, significantly more disease occurred on plants stressed from an imbalance of either nitrogen, potassium or phosphorous than when optimum levels of each nutrient were available to plants. Severity of black dot was also greater when plants were stressed by excessive irrigation than when plants were optimally watered.

Infection of potato tubers by the black dot fungus results in the development of blemishes or superficial lesions on the tuber surface (Fig. 5). Sclerotia usually develop in the lesions when humidity is high. Symptoms are commonly observed at the stem end of the tuber and the lesions can appear silvery to brown, generally with a poorly defined margin. In contrast, silver scurf lesions are silver, with a clearly defined margin. Confirmation of these two diseases requires examining tubers with a hand lens or microscope to observe the characteristic sclerotia of black dot, or spores of the silver scurf fungus.

Black dot symptoms of blemishes and lesions are usually not present on the tuber periderm at harvest, but they later develop in storage. This indicates that the tubers are latently infected in the field before harvest. In studies in the Columbia Basin, incidence of black dot significantly decreased as the number of years between potato crops increased (Figs. 6).

A sharp decline in incidence of black dot occurred after three years out of potato production. Black dot decreased 10 percent per increasing year between potato crops. The black dot fungus was sometimes detected in fields out of potato for 10 and 15 years, but incidence of detection of the fungus was generally low after five or more years out of potato production (Fig. 6). The incidence of black dot was also low with a few number of previous potato crops (Fig. 7). Both factors, years between potato crops and number of previous potato crops, accounted for 87 percent of disease incidence for black dot (P < 0.0001, R2 = 0.87) with multivariate regression analysis. Weed hosts may account for long-term persistence of the fungus is soil.

Management of Black Dot

No single management tactic will successfully eliminate the effects of black dot on potato. As a result, several tactics must be integrated to successfully manage the disease. Lengthening the time between potato crops reduces the effect of soil-borne inoculum and can be a helpful disease management tactic.

Data from recent studies suggest that more than four years out of potatoes are needed to appreciably reduce the effects of the disease from soil-borne inoculum. An application of a strobilurin at 40-62 days after planting (growth stage of plant foliage touching within rows to touching between rows) can significantly reduce black dot sclerotia in upper and lower stems and tubers, which may reduce soil inoculum in subsequent years.

Even though soil-borne inoculum has a higher potential in causing severe disease than tuber-borne inoculum, potato seed lots with a high incidence of black dot should be avoided. Recent research demonstrated that C. coccodes grows from infected seed piece to roots, stolons and tubers at a rate of about 1 mm/day.

The fungus can colonize roots, stolons and reach daughter tubers from an infected seed piece during a long growing season. Reducing plant stress is an important management tactic for black dot. A balanced plant nutrition program is important in reducing the effects of black dot on plant growth. Adequate nutrients may especially be important going into the bulking phase of plant growth. Plant stress from soil compaction and over-irrigation compaction should be avoided. Not only does water-saturated soil favor spread and development of C. coccodes, but oxygen is displaced in the soil, which is needed for oxidative respiration by roots.

Protecting young plants from blowing sand, which may increase the incidence of foliar infections, should be avoided. Co-infection with other pathogens, especially the Verticillium wilt fungus will increase damage in many potato cultivars. Consequently, black dot management also includes managing Verticillium wilt and other potential causes of early crop death. PG