NCSU

Colletotrichum coccodes

By Emily Silverman

A Class Project for PP728 Soilborne Plant Pathogens
North Carolina State University
Department of Plant Pathology

Black dot of potato is caused by Colletotrichum coccodes

Introduction:

Black dot of potato, caused by C. coccode, is a common disease of potato. C. coccodes is a cortical root rotter. C. coccodes mainly attacking tubers but other plant tissue can be infected such as stems, roots, and stolons. The disease can cause early plant decline leading to discolored tubers and eventually yield reduction.

Host Range and Distribution:

C. coccodes has a large host range. Solanaceous species are susceptible to C. coccodes. Tomato is infected by this pathogen and the disease is anthracnose fruit rot. Cucumber, legumes, mint, peppers and weeds are common hosts of the pathogen. Nightshade, a weed species commonly found in agriculture production systems, has been observed to harbor the fungus and aid in overwintering.

Scotland, England and Ireland have recurrent issues with black dot in potato production. There are many regions in the United States were potatoes are grown. Maine, New York, and Pennsylvania were large production area for potatoes in USA but production shifted to the western part of the country and now Washington, Idaho, Colorado, and Nebraska grow most of the USA’s potatoes. Canada, Mexico, Germany, Netherlands are large potato producing nations as well. Wherever potatoes are grown black dot is a problem.

Coarse textured soils with low/excessive fertilization, high temperature and poor drainage are conducive for C. coccodes. Cold locations with long winters and extended low temperatures are preferred for potato production.

Identification:

The presence of black dots on tuber tissue is a distinguishing characteristic from other tuber diseases. The black dots are sclerotia. Stem infections result in a white mass of mycelium that produces sclerotia on senescent tissue. Stolons adhering to tubers can be an indication of infection but the visible sclerotia are a sure sign of the fungus. The above ground symptoms can be identical to other wilt pathogens like Fusarium or verticillium wilt.

 Microscopic examination of fruiting structures reveals conidia that are cylindrical, hyaline and aseptate (16-25 μm x 3-4 μm) formed on cylindrical conidiophores. Acervuli on the roots are rounded (diam. 150-300 μm) with septate setae on the surface.  Appressoria are ovate to elliptical (av. 12 x 7 μm) in shape. Mycelium are dark in pigmentation.

Setae and conidia

Description:Setae (left) on the surface of plant tissue give rise to conidia (right). Photographs courtesy of Cornell University, Department of Plant Pathology.  Photographer: Kent Loeffler.  

Isolation:

Isolate sclerotia from soil samples on SST media. Mix equal parts quartz sand, garden loam sand, and ground tomato roots from mature plants. Autoclave for 45min, adjust moisture holding capacity to 80% and reautoclave for 15min. Sclerotia can also be produced on V-8 juice agar: 30% V-8, 3% agar, pH4.6. Incubate plates in the dark for 2 months at 22-25C.  Sclerotia can also be produced on PDA media. Cover a cellophane layer over a PDA plate and place inoculum on the surface of the cellophane and incubate for 1-2months . Scrape the surface of the cellophane to collect sclerotia and grind them in water containing fine mesh sand and rinse through a 150-250um sieve. Conidia can be grown on 30% V-8 juice agar amended with celite at pH4.5 in constant light at 23C.

Symptoms and Signs:

The disease is named for its characteristic black dots which are sclerotia. Sclerotia are visible on the surface of plant tissue above and below ground. Roots, shoots, stolons and tubers may have black dots indicating infection. Infected roots have lesions brown to black in color and root growth is stunted. Lesions are present at the onset of disease development brown in color and small in size slowing coalescing. Rhizoctoni stem infections look very similar and can be hard to differentiate from black dot lesions. A distinguishing characteristic of black dot infected tubers is the adherence of stolons to the stem end of tubers. Infected tubers may be brownish gray in color covering a large area of a tuber when infected by C. coccodes. The discoloration of tuber skin is common with silver scurf, another disease of tubers. Black dot sclerotia form on the discolored infected tuber tissue and are visible with a hand lens. Long term storage of tubers can result in discolored tubers similar to silver scurf. Sclerotia are made up of acervuli, black long needle-like structures that form setae and produce conidia that can disseminate to cause infection. They form at the end of the field season and facilitate overwintering of the fungus on plant debris such as stems, stolons, roots, shoots and tubers.

Foliar disease symptoms are similar to early blight brown flecks. Yellowing and wilting occur mid to late season and wilt is rapid. These general symptoms can easily be confused with other diseases such as early blight, Fusarium wilt, and Verticillium wilt.

stolon adhesion

Description: Adhesion of stolon to tuber is common with black dot on tubers

wilt

sclerotia

Description: Stem infection with visible sclerotia

Photo courtesy of University of California Statewide IPM program. Photographer: Jack Kelly Clark

Description: Above ground symptoms include wilt, foliar yellowing and brown flecking of foliage

Photo courtesy of University of California Statewide IPM program. Photographer: Jack Kelly Clark.

Photo courtesy of University of California Statewide IPM program. Photographer: Jack Kelly Clark

stem infection

Description: Lower stem infection of black dot start as small brown lesions that coalesce and sporulate on senescent tissue.  Photo courtesy of Willie Kirk, Michigan State University, Department of Plant Pathology

Epidemiology:

The fungus overwinters on debris in the field, as sclerotia on tubers and can be in reservoir hosts such as other solanaceous crops and weeds. C. coccodes is not a soil inhabitant, it can survive in soil but requires crop debris for survival. Conidia are the main infectious propaguales of the fungus that disseminate in spring from acervuli. Secondary infections of conidia throughout the season are possible under favorable conditions. Stressed plants are more vulnerable to infection such as extended dry hot weather.

Conidia are windborne, splash dispersed, and can be spread by soil movement and irrigation water. Conidia production is favored in high temperatures between 45-95F. Conidial germination is favored by the presence of water on plant surfaces. Conidia enter through wounds and can directly infect epidermal tissue.

disease cycle

Description: Life Cycle of Colletotrichum coccodes. Image courtesy of Willie Kirk, Michigan State University, Department of Plant Pathology. Illustrated by Marlene Cameron.

Control:

Proper site selection is important but improving drainage can help reduce disease in situations where the site is already selected and crop is infected. Use of clean tubers for propagation material is suggested because contaminated seed pieces are a common means of spread of the pathogen. Removal of weeds around fields can reduce disease incidence as well. Removal of crop debris and culling of tubers from fields also can be used to manage the pathogen.

Deep plowing promotes degradation of infected plant debris by burial of infectious propagules and can help effectively reduce fungal populations. Mouldboard plowing at depths of 30cm is sufficient to reduce but 60cm has been observed to be less effective. Soil solarization has also been proven to reduce disease incidence of black dot. Disease incidence was reduce by 45% with soil solarization for eight weeks. Temperatures reached a maximum of 56C in the top few centimeters of soil. Six weeks of soil solarization resulted in 25% disease reduction of black dot as observed by Denner et al. 2000.  

Crop rotation for 3-4 years without potatoes or other solanaceous species is required. Small grain, soybean, or corn can be used in rotation to reduce fungal populations. C. coccodes can survive in the soil for up to 8 years. Cucurbits and legume species are also severely affect by C. coccodes and should not be used in a crop rotation program.

There are no resistance cultivars in use but planting early may help avoid the disease because it is a late season pathogen. Russet cultivars are less susceptible to the disease compared to thin skinned varieties.

REFERENCES:

 1.   Pavlista, Alexander D.; Kerr, Eric D.; and O'Keefe, Robert B., "G92-1090 Black Dot Disease of Potato" (1992). Historical Materials from University of Nebraska-Lincoln Extension. Paper 1266.

2.    TerraLink. Pest Bulletin. May 2011.

3.    Golzar, Hosseln. Pathogen of the Month: Collectotrichum coccodes. June 2009.  American Phytopathology Society.

4.    United States Potato Board. 2007.

5.    Dhingre, O.D., Sinclair, J.B. Basic Plant Pathology Methods. Second Ed. Lewis Publishers. Baco Raton, FL. 1995. P21.

6.    UC Plant Management Guidelines. Potato: Black Dot. University of California Statewide IPM Program, Agriculture and Natural Resources. Revised: 3-16-12.

7.    Phillip Wharton and William Kirk. Michigan Potato Disease: Black Dot. Department of Plant Pathology, Michigan State University. Revised: 9-24-12.

8.    Vegetable MD Online, Black Dot Disease of Potatoes.Cooperative Extension, New York State, Cornell University. Fact Sheet Page 725.70. Date 8-1989.

9.    Denner, F.D., C.P. Millard, F.C.Wehner. Effect of soil solarisation and mouldboard ploughing on black dot of potato, caused by Colletotrichum coccodes. Potato Research. 2000. 43:195-201.