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Potato Disease Management in 2001

Thomas A. Zitter
Department of Plant Pathology
Cornell University
Ithaca, New York
Issued 4-20-01

The 2001 potato growing season is rapidly approaching, so here are a few pointers to keep in mind to get your season off to a good start. Late blight occurred only sporadically on potato last year in New York, and from most accounts, we should be heading into the season with a low level of inoculum. The US-8 (potato pathotype) did occur last year, but expedient use of fungicides kept spread to a minimum. But this is no time to get complacent, since prevention is still the key word for potato disease management strategies.

Destroy cull piles and volunteers. Growers are well aware of the importance of cull piles as potential inoculum sources. Remember that tubers in stone piles at the edge of fields can also be a problem. We are not aware of any late blight in the state as a carryover from 2000, but because volunteers are likely to be among the earliest sources of late blight inoculum in the field, it is essential that volunteers be destroyed as soon as they are seen. The goal is to prevent the development of green leaf tissue and subsequent infection and sporulation of the late blight fungus. Fields with volunteers should be plowed as early as possible after potatoes emerge.

Seed tubers and treatments. We have had no reports of late blight being found in potato tubers coming out of winter storage here in New York. However, given that late blight occurred in some seed production regions of the US in 2001, we could expect that inoculum will be present in some seed lots. If you have any doubt about the quality of your seed, it would be smart to examine the seed closely, not only for the potential of late blight, but also for the presence of Fusarium (the dry rot fungus), Rhizoctonia (the black scurf fungus), Helminthosporium (the silver scurf fungus), and powdery scab (Spongospora subterranea). Make sure to cut some of the seed in advance, since seed may appear good on the outside, but internally it may have problems. The occurrence of Fusarium dry rot can prevent the proper healing of cut tubers, and this allows the easy invasion of bacterial soft rot, and will result in a very poor stand. The other seedborne pathogens are sometime hard to see on seedpieces, and therefore a seedpiece treatment is recommended. The adverse affects of Fusarium dry rot was most obvious in our potato trials last year, and seedpiece treatments really came in handy in our cold and wet spring soils. Seedpiece treatments are especially important if you are cutting and healing (suberizing) your seed before you plant. The seedpiece options available in New York are limited when compared with other states, and are listed in the current Vegetable Guidelines. The material(s) you choose will depend upon the quality of your seed and even your potential market for future sales, since some seed treatments have a major affect on daughter tuber infection (especially for black scurf and silver scurf). Captan, maneb and mancozeb are traditional products used for Fusarium dry rot control. The addition of mancozeb to TOPS MZ and Maxim MZ has also been shown to prevent the spread of late blight from surface infestations when seedpiece to seedpiece contact occurs. In our trials last year, both Maxim MZ and TOPS MZ also reduced the amount of Rhizoctonia and silver scurf found on daughter tubers. Our most effective treatment for control of these two diseases was an in-furrow treatment using Quadris. Unfortunately, this usage is not currently available for use in New York.

Scout cull piles, volunteers, and emerging fields. If you are not sure that you can recognize late blight, obtain the Vegetable Crops Fact Sheet 726.20 authored by William Fry in July 1998, with colored pictures of the symptoms of late blight of potatoes and tomatoes. You can also find this at the Vegetable Mdonline web location or at this specific site: http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm

Scout aggressively for signs of late blight, beginning as soon as green tissue emerges. Late blight can occur on stems as well as leaves. When a canopy develops, look for blight in the lower portions of the plant where the foliage stays wet. Take care not to spread late blight from field to field when scouting. Rubber (or disposable) pants and boots, which can be washed after leaving a field, should be used if disease is present. Inoculum also can be carried from field to field on equipment, but is easily removed by washing with water.

Fungicide options for late blight and early blight. We can expect that most inoculum of late blight, when it occurs in the state, will be of the new immigrant genotypes which are Ridomil-resistant, and are more aggressive than the older isolates. Therefore, growers should not depend on Ridomil for late blight control. The key for late blight control is to use any of the following materials in a preventative manner. The protectant fungicides most commonly used are chlorothalonil (Bravo, Equus, and others), mancozeb (Manzate 75DF, Dithane DF, Penncozeb75DF, and Manex II), maneb (Maneb 75DF or Manex), metiram (Polyram 80DF) and triphenyltin hydroxide (Super Tin 80WP). Triphenyltin hydroxide plus metiram or a mancozeb product are often used for mid-season multiple-disease control when disease pressure is less severe. Bravo Weatherstik has long been a standard, and if used in very early disease development can be quite effective. The newer formulations of Dithane DF Rainshield and Manzate 75DF will provide growers with good control at times of less intense disease pressure.

When conditions are most favorable for late blight, or if Phytophthora has been reported in the area, two additional products should be considered. Both Quadris and Curzate have preventative, curative and locally systemic activity, but if disease is already present, other products like Tattoo C (now called Previcur) have performed better. It is important to follow label directions to prevent fungicide resistance from developing for both Quadris and Curzate. Quadris (azoxystrobin) will provide some control of late blight and excellent control of early blight. For potatoes Quadris is normally used at the rate of 6.2 fl oz/A, but should be increased to 12.4-15.4 fl oz/A if late blight symptoms develop or if conditions favor disease development. Quadris should be used in a strict one to one alternation with a fungicide with a different mode of action such as chlorothalonil or another protectant material. Curzate 60DF is registered for use on potato at the rate of 3.3 oz/A and must be tank mixed with a protectant fungicide such as mancozeb, chlorothalonil, or other protectant fungicide listed above. Even when tank mixed with a protectant fungicide, it is recommended that the spray schedule using Curzate be switched to a different fungicide program with another mode of action after several applications. NOTE: Curzate alone (as it should never be used) does not control early blight.

Unfortunately, neither Acrobat MZ (dimethomorph + mancozeb) nor Previcur (propamocarb, formerly Tattoo) are registered for use on potato in New York at this time. Also a new product from Rohm and Haas that is quite effective for late blight control, called Gavel 75DF, will not be registered in time for this season. We are working to get these needed products registered in New York.

For organic growers, several fixed copper fungicides are available (Basicop, Champ, Kocide, etc.) and provide fair control of late blight and early blight.

Reporting late blight. County agents/specialists and consultants have been asked to report the occurrence of late blight, on a county-by-county basis. This information will be made available to growers so that spray schedules can be modified to reflect inoculum availability. It is also possible to analyze the genotype of the blight organism in your field (or on seed tubers). Contact your county agent/specialist to make these arrangements.