Overlooked Diseases: Part II

Gray mold, black dot and white mold

Published in the May 2009 Issue Published online: May 03, 2009 Steve Smede
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As noted in the last issue of this magazine, growers are all too aware of certain diseases, such as Late Blight, PVY and PLRV. But what about the scores of other microscopic monsters that get far less attention?

Based on five years of plot work at the Aberdeen Research and Extension Center in southern Idaho, soil and disease management specialists have presented an overview of their studies focusing on a handful of lesser known pathogens. Presented by Phil Wharton, Jeff Miller and Phil Nolte at the 2009 Potato Conference in Pocatello, Idaho, the roster includes Rhizoctonia, Black Scarf, Powdery Mildew, Early Blight, Brown Leaf Spot, Gray Mold, Black Dot and White Mold.

The April edition of Potato Grower focused on the first three; this issue will address White Mold, Gray Mold and Black Dot.

Extensive research on these diseases is ongoing, especially as they pertain to production agriculture. To varying degrees, potato crops are affected by each of them. Specialists in studying the pathogens and control methods include teams at the University of Idaho, the Plant Disease Diagnostic Clinic at Cornell University and the agricultural research departments at Oregon State University, University of California and University of Nebraska Lincoln.


White Mold, brought on by the soil-borne fungus Sclerotinia sclerotiorum, has been closely studied by Nolte (an Extension Seed Potato Specialist) and Miller (a Potato Pathologist) as well as UI Extension Potato Specialist Nora Olsen and Terry Miller-a Plant Pathologist with Miller Research in Rupert.

Potatoes are certainly not alone in hosting this disease. More than 400 other species are on the target list, including beans, peas and canola. A few weeds also host the pathogen, including lambsquarters and pigweed.

This fungus over-winters in the soil in the form of hard, black quarter-inch structures. Near the surface, these structures can germinate, producing small "mushroom-like discs," which (much like other diseases) favor cold temperatures and moist soil.

According to Miller Research and the UI group, germination usually occurs after row closure and may last for a period of two to eight weeks. They note that under favorable conditions, spores can be dispersed by wind throughout an entire field or to adjacent fields. Up to 8 million spores can be produced by a single disc.

Consistent with the pathogen's preference for moist conditions, one of the first signs of this disease is "water-soaked lesions" that are covered by fungal growths resembling white cotton. Affected plant stems will become wilted. (Lesions only rarely form inside tubers.) Eventually the stems dry out and the plant dies, releasing the pathogen back into the soil and starting the cycle of disease anew.

Due to the proliferation of spores and its longevity in the soil, white mold can work its way from an obscure pest to a significant canopy killer, which presents concern for overall yield potential.

For management practices, Miller Research and the UI specialists encourage an integrated approach to reduce inoculum in the field. Specifically, that means rotations with weaker or less susceptible hosts (corn, small grains). White mold's spores will still do their work, but when faced with less attractive host plants, the disease's life cycle will be jammed. (Rotations to avoid: beans, alfalfa and canola.)

Other control suggestions include the following:

  • Reduce excessive vine growth for a thinner canopy.
  • Avoid excessive irrigation whenever possible to help reduce excessive humidity and wetness in the canopy.
  • If possible, target weed-control efforts to plants that are susceptible to the disease.

Similar in some respects to White Mold is Botrytis cinerea-a fungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as "botrytis bunch rot;" in the parlance of western U.S. agriculture, it is usually called "Gray Mold," and it is known to affect a number of plants, including potatoes. Like white mold and a number of other diseases, this pathogen finds comfort in prolonged periods of cool, rainy weather in the first half of the growing season.

According to The Plant Disease Diagnostic Clinic at Cornell, during wet or humid weather is the time to examine plants for any brown or spotted material that develops, as well as any masses of silver-gray spores on the dead or dying tissue.

On the most heavily infected plants, spores are as easy to spot as dust. Some species also form tiny black structures (called sclerotia) on dead tissue by late summer.

For certain species of Gray Mold, compact fungal masses develop in the dead plant tissue and-similar to the sclerotia of White Mold-form the over-wintering stage of the disease. These masses then germinate in the spring, or the vegetative part of the fungus grows out of infected material, developing spores. Water spray, rain and/or wind can all work in tandem to spread infection to more plants.

According to The Plant Disease Diagnostic Clinic at Cornell, the best way to manage these diseases is by vigilant inspection and sanitation. For chemical control options, contact university extension personnel for the most recent list of registered products and recommendations.


Another often-overlooked gremlin in some potato-producing regions is Colletotrichum coccodes. This fungus, known colloquially as Black Dot, forms many minute, black sclerotia on roots, stolons, tubers and stems-especially late in the growing season. The dreaded result: root and stolon necrosis.

According to plant specialists at Oregon State University, the signs are hard to ignore:

  • Leaves may progressively yellow and wilt.
  • The stem yellows, browns and dries.
  • Sclerotia appear at the base up to several inches above soil level.
  • Dark, necrotic, sunken lesions may be found on stems, petioles and veins.
  • Lesions may advance, causing the affected stem to die back or completely wilt.

For all its frightening effects, researchers still consider Black Dot to be "a weak pathogen considered a minor disease in potatoes," if only because its symptoms are so easily confused with those of other ailments.

Yellowing and wilting of plant tops will be apparent by late summer. Shortly thereafter, plants will turn brown and die. These symptoms are easy to get confused with similar effects of Fusarium and Verticillium, but the key difference to watch for is the black dots themselves.

Measuring just 1/50 of an inch or so, these fungal masses are quite properly described by OSU researchers as "small black sea urchins under magnification. The most striking symptom appears below ground on the stem. When the outer tissues (cortical scales) are peeled back, the exposed woody vascular tissue turns reddish or amethyst color. The fungus produces the black dots on both the inner and outer surfaces of the stem. Necrotic lesions occur on the underground stems, roots and stolons, and pinhead-sized black sclerotia, the dots, can be seen on the affected areas."

Ultimately, the dismal effects on stems and roots spells an early doom for the plant, or at the very least-a reduction in tuber size.

According to R.M. Davis, et al, of the University of California statewide integrated pest management program, Black Dot occurs most frequently on plants grown in "coarse-textured soils under conditions of low or excessively high nitrogen, high temperature, or poor soil drainage."

The thicker-skinned tubers of Russet cultivars are less severely affected by this disease, as are early-season varieties.

According to reports from OSU, the pathogen moves with the wind but is first introduced to the soil via infected seed tubers.

Like so many other infestations, black dot goes after the weakest plants first, attacking aging tissue, or injured or stressed plants. Like White Mold and Gray Mold, it can also over-winter in debris left in the field.

There are no means of chemical control specifically for black dot, but two- to three-year crop rotations with grains can certainly help by reducing the inoculum in the soil. Adherence to good irrigation practices and fertility management will help avoid crop stress and susceptibility. And of course, it always helps to plant certified seed tubers.

As any veteran grower or longtime crop specialist will attest, there are no guarantees in spotting and dealing with diseases. This is true for the most devastating fungi in the limelight, but especially so for diseases that are often overlooked or misidentified. However, with some added background knowledge and a little extra diligence in monitoring and managing the field, these shifty pathogens will find it harder and harder to negatively affect that all-important bottom line.

Sources: "Use Of Foliar Fungicides For Managing Potato Diseases," by Jeff Miller and Terry Miller; research information on Gray Mold from the Plant Disease Diagnostic Clinic at Cornell University; background information on Black Dot from Oregon State University, University of California's Statewide Integrated Pest Management Program and additional background information from the University of Nebraska Lincoln.


Overlooked Diseases, Part I and II Recap:

Rhizoctonia - The most conspicuous sign is black scurf. Look for stem girdling and collapse, stolon girdling and aborted tubers.

Powdery mildew - Almost completely on the outside. Likely to pop up in areas where a grower may still be using flood irrigation or areas prone to high humidity.

Early blight - Often misidentified with brown leaf spot. The most obvious sign of early blight is the characteristic banding pattern on the lesions of an infected plant.

Gray mold - Compact fungal masses develop in the dead plant tissue and form the over-wintering stage of the disease. These masses germinate in the spring, or the vegetative part of the fungus grows out of infected material, developing spores. Water spray, rain and/or wind can spread infection.

Black dot - Forms many minute, black sclerotia on roots, stolons, tubers and stems-especially late in the growing season. Sclerotia appear at the base up to several inches above soil level. Dark, necrotic, sunken lesions may be found on stems that may advance until stem dies or completely wilts.

White mold - Over-winters in the soil in the form of hard, black quarter-inch structures. These structures can germinate, producing small mushroom-like discs that favor cold temperatures and moist soil.

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