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Update on Pink Rot
and Pythium Leak Control for Potatoes
Thomas A. Zitter
Department of Plant Pathology
Cornell University
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.
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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.
Varietal Susceptibility
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 season’s 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, it’s 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 product’s 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.
Fungicide Resistance
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.
- Select areas with well-drained soils to avoid conditions favorable for both
pink rot and leak.
- Use crop rotations out of potatoes for at least 4 years. Even one year out
of potatoes will drastically reduce soil fungal populations, as the previous
potato crop produces most pink rot inoculum.
- The level of resistance to pink rot is predictable from tuber isolations.
By having tubers tested, growers can differentiate between resistance and
lack of control due to application or persistence problems.
- Pythium leak invades the tuber through wounds that occur during harvest.
Avoid harvesting potatoes when soils are especially wet or when flesh temperatures
are high (> 68 F), as leak development under these conditions is very rapid.
Adjust harvesting equipment to keep digger chains fully loaded and minimize
drops to 6 inches or less.
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| Fig.1 |
Fig. 2 |
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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