Thomas A. Zitter
Department of Plant Pathology
Ithaca, NY 14853
Issued June 2002
Pink rot, caused by Phytophthora erythroseptica, and leak or shell rot, caused by Pythium spp. (primarily P. ultimum), are two important soilborne pathogens that directly affect tuber integrity in storage. P. erythroseptica (Fig. 1) invades the potato plant through the roots, and mainly infects the tubers at the stem end through stolons, though infection may also occur through harvesting wounds, lenticels, and buds. Pythium primarily invades the tubers through harvesting wounds, and post harvest rot often develops in transit or in storage (Fig. 2). Excessive moisture (poor drainage, excessive precipitation or irrigation) plays a key role in tuber infection for both diseases. Further losses may occur in storage due to secondary bacterial infections of the damaged tissue.
Although we have limited direct information on the occurrence and performance of selected fungicides for the control of pink rot and Pythium leak in New York, we can draw upon some valuable information from other states, Canada, and Australia. Most of the information presented here is from efforts to control pink rot, but these findings also apply to leak occurrence and control.
Most potato varieties should be assumed to be susceptible to either pink rot or leak, but research has shown that some varieties are particularly susceptible, especially to pink rot. These varieties include Russet Norkotah, FL 1533, Red LaSoda, Dark Red Norland (often used as a susceptible standard), and NorDonna.
How Is Tuber-Susceptibility to Pink Rot/Leak Evaluated?
Testing for susceptibility to pink rot/leak can be accomplished by either inoculating field plots with inoculum grown on vermiculite or sand, and then distributing it into the furrow or broadcasting it, or by direct inoculation of stored tubers at seasons end. The former method has been used to evaluate fungicidal control of both diseases in the field, while the latter method has been used to evaluate varietal susceptibility to pink rot (without a specific fungicide treatment in the field), and test the efficacy of field-applied fungicides (either in furrow or foliar application) prior to experimental storage inoculation. Experimental inoculations are accomplished by wounding whole tubers with a nail at the stem end (stolon attachment) or by making a series of shallow wounds across the tuber surface, just prior to the introduction of a droplet of inoculum. Alternatively, a plug of potato tissue can been removed from the opposite end of the stolon, a mycelial plug of P. erythroseptica inserted into the hole, and the tissue plug replaced into the tuber. Following an incubation period, the tubers are cut in half, exposed to air at ambient temperatures, and assessed for P. erythroseptica or Pythium symptoms, which are confirmed as positive by either specific ELISA assays or plating onto selective media.
Fungicide Usage and Evaluations
Most studies have involved the use of mefenoxam, which is the active ingredient in Ridomil Gold or Ultra Flourish, and are applied as a 6- to 8-inch band directly over the seed piece prior to row closure at the rate of 0.42 fl oz 4E/1000 feet of row or 0.84 fl oz 2E/1,000 feet of row, respectively. The in furrow treatment is now widely used in the Midwest and Maine. Alternatively, Ridomil Gold Bravo or Flouronil (mefenoxam/chlorothalonil) at the rate of 2 lb 76WP/A (or Ridomil Gold MZ at 2.5 lb 68WP/A) can be used as a foliar spray with as much gallonage as possible. The first spray is applied at flowering (nickel size of setting tubers), with a second and third sprays applied 14 and 28 days later. The third spray would be necessary only for fields with a history of storage rots. When applying these products as foliar sprays, its important to achieve coverage of the soil surrounding the plants. Research has shown that mefenoxam moves systemically within a plant, and up to 96% enters through the stolons. This implies that not much moves down the stem and therefore foliar applications are better if they are washed into the soil. The downside of using foliar application and allowing irrigation to move the product into the soil is that the entire fungal population is exposed to the products active ingredient. This may be a factor in the development of resistance as discussed below.
One trial showed Ultra Flourish combined with phosphorous acid achieved better control than with either Ultra Flourish alone, or from another study when phosphorous acid was used alone. Less effective materials tested include Zoxium (zoxamide), Omega (fluazanim), and Acrobat (dimethomorph), all of which have activity against late blight, caused by P. infestans. Currently only products containing mefenoxam and Phostrol (mono- and dibasic sodium, potassium, and ammonium phosphites) are registered for use on potatoes in New York.
The activity of mefenoxam depends upon its availability in soils and this can be diminished in soils with higher organic matter, sand or sandy loam soils, and from frequent use of irrigation, since mefenoxam is very soluble. Microbial breakdown also proceeds faster at low pH, which can occur when fertilizers are mixed with mefenoxam. The half-life of metalaxyl in soil is generally considered to be 36 days, but some accounts report a half-life approaching 72 days for mefenoxam. This longer half-life may explain the good control obtained when mefenoxam is applied in furrow.
Resistance of the fungi causing pink rot and leak to metalaxyl (Ridomil) and mefenoxam has been reported from several locations in the US and Canada, but the majority of the population remains sensitive. Resistance to pink rot has been noted from Idaho to Maine and east to New Brunswick, while a previous report of resistance in New York was caused by a lack of product activity. Resistance of metalaxyl and mefenoxam to leak has been confirmed in Minnesota and Idaho. Long term survival of moderately resistant isolates of P. erythroseptica in Maine indicates that this population maintains its fitness in the soil environment equivalent to the sensitive population, suggesting that steps need to be taken each season to prevent or reduce the occurrence of resistance. Studies from South Dakota and Australia suggest that mefenoxam resistance can be carried on soil that adheres to seed potatoes.
How to Prevent/Manage Fungicide Resistance?
Cultural practices play a big role in preventing or managing resistance to mefenoxam.
Fig. 1. Pink rot, caused by Phytophthora erythroseptica, results in rotten and discolored periderm that remains intact, and the margin between healthy and diseased tissue if often delineated by a dark line visible on the tuber surface. The internal flesh turns salmon pink after exposure to air for 15-20 min, and then turns brown to black. (Photo courtesy of R. P. Mulrooney, University of Delaware).
Fig. 2. Leak, caused by Pythium spp. (P. ultimum), appears internally as a lesion with a sharply defined reddish-brown to black border surrounding gray or brown watery rotted tissue. Squeezing the tuber enhances the appearance of the watery condition. (Photo courtesy of T. A. Zitter, Cornell University).
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