Phillip Wharton and William Kirk
Department of Plant Pathology, Michigan State University
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Pink rot of potato, caused by Phytophthora erythroseptica,
is an important storage disease of potatoes worldwide. The disease is
usually associated with high soil moisture when tubers are approaching
maturity and is a serious problem in poorly drained soils. Although
the disease is predominantly found in wet fields it can also develop
in sandy soils without excessive moisture. Pink rot was first reported
in the United States in Maine in 1938 and has become widely distributed
in North America. Potato growers refer to the disease as "water rot"
without distinguishing the pathogens involved. The name pink rot describes
the pink color that develops in infected tuber tissue when tubers are
cut and exposed to air for 15 to 30 minutes (Fig. 1). Pink rot is a
serious problem in storage and high humidity along with poor ventilation
can cause heavy losses of stored potatoes. Pink rot infection is usually
associated with secondary infection by anaerobic soft rot bacteria (Fig.
Diseased plants are first observed in poorly drained parts of the field. Disease symptoms, mostly characterized by stunting and wilting, appear on the potato vines late in the growing season. Wilting starts from the base of the stem and progresses upwards causing leaf yellowing, drying and defoliation. Vascular discoloration and blackening of the underground stems may also be observed. Similarly roots may turn brown to black in color and occasionally aerial tubers may develop.
Symptoms on tubers are more obvious and characteristic
of the disease. Tuber symptoms are characterized by a tuber decay that
begins at or near the stem or stolon end of the tuber. Infected tissue
becomes rubbery but not discolored in the early stages of infection
and when infected tubers are cut open the rotted portion is delimited
by a dark line, visible through the tuber skin (Fig. 3). The tuber skin
(periderm) over the rotted portion is light brown in color in white
skinned cultivars. Pink rot is not a slimy soft rot and rotten tissues
remain intact but spongy. When rotten tubers are cut open, the internal
tissues are cream in color and usually odorless. The tough, leathery,
rubber-like texture of infected tubers distinguishes pink rot from bacterial
rot diseases in which the diseased tissue becomes soft, pulpy and contains
numerous cavities. However, infected tissues are easily invaded by secondary
pathogens, such as soft rot bacteria (Pectobacterium spp.) which
produce the slimy symptoms often found in potatoes with pink rot (Fig.
2). As tubers are exposed to the air, the color of the infected tissue
progressively changes color from cream to salmon pink within 15 to 30
minutes (Fig. 1). After about one hour the tissue gradually turns brown
and then black. If the cut tuber is squeezed, a clear liquid may ooze
out of the cut surface.
Phytophthora erythroseptica, the causal agent of pink rot is
a soil born pathogen that produces thick walled sexual spores called
oospores that can survive and persist in the soil for up to seven years
(Fig. 4). Soil borne oospores are the primary source of inoculum for
pink rot in potato. Transmission by infected seed tubers has always
been considered of minor importance since rapid rotting and decay of
tuber tissue during storage will exclude these tubers from being used
as planting material. However, the surface of healthy tubers may be
contaminated with oospores that can be transferred to new locations.
Oospores formed in infected tubers missed during harvest that remain
unfrozen over the winter (volunteer potatoes) or in cull piles will
end up in the soil after deterioration of the tubers (Fig. 5). The pathogen
can also be transferred to new fields in infested soils carried on farm
machinery and bins. Within an infested field oospores are likely to
spread when soil is moved around during cultivation. Irrigation water
is also an important source of movement of the oospores from one location
to another within a field and among nearby fields. Pink rot may also
spread from tuber to tuber in storage.
Phytophthora erythroseptica, develops rapidly
at soil temperatures from 50 to 86°F. However, the optimal temperature
for infection is 77°F. The pathogen usually infect tubers through stems
below soil level and stolons. In warm, wet conditions oospores of P.
erythroseptica will germinate and produce the asexual stage of
the pathogen which consists of mycelia and sporangia. Infection occurs
when germinating oospores, sporangia or zoospores penetrate the epidermis
of underground portions of the plant. Sporangia are the secondary source
of inoculum for dispersal of the pathogen in the field or storage. Sporangia
germinate producing either a germ tube that can penetrate and infect
new tubers or produce zoospores. Zoospores swim freely in water films,
attach to the host surface (encyst), and infect the plant. Roots, stem
bases, stolons and tubers may become infected. In water logged soils
infection can also occur via tuber eyes and lenticels. Disease incidence
is usually higher in water-logged areas of a field.
Although initial infection occurs during the early stages of tuber development the disease becomes most apparent during harvest. Tubers infected with pink rot in the field will rot during harvest and handling allowing the pathogen to come into contact with healthy tubers. Wounds and bruises on tubers caused by harvesting and handling provide entry points for the pathogen into healthy tubers and thus postharvest rotting is usually more severe than field infection. In moist storage conditions with poor ventilation the disease spreads rapidly.
No single control measure will provide effective control of pink rot. However the disease can be managed using an integrated approach that combines the use of host resistance, cultural and chemical control methods.
One of the most successful and important approaches to pink rot control
is to reduce the amount of inoculum in soil by removing crop debris,
volunteer and cull potatoes from the field. Crop rotation with non-host
plants can also reduce the amount of inoculum in the soil. A three to
four year rotation including legumes is useful to reduce the build up
of P. erythroseptica inoculum in the soil. Soil moisture management
is also a significant approach to the management of pink rot. Improving
the drainage of potato fields and adjusting irrigation systems so water
does not accumulate in parts of the field will certainly reduce the
incidence of pink rot. The following cultural practices are also suggested
to prevent and reduce the incidence of pink rot:
- Avoid excessive soil moisture especially late in the growing season.
- Let tubers establish good skin set before harvesting.
- Avoid wounding and bruising of tubers during harvest.
- Avoid harvesting tubers from poorly drained parts of the field.
- Avoid harvesting tubers when the pulp temperature is above 65°F.
- Remove infected tubers at harvest prior to piling.
- Cure tubers at 45 to 50°F at > 90% relative humidity to promote
- Reduce tuber pulp temperature in storage to 50°F or lower as quickly as possible.
- In storage, use high airflows and prevent water condensation in the pile.
Most commercially grown potato cultivars in the United States are considered to be susceptible to pink rot. Limited breeding has been done to developed potato cultivars that are resistant to pink rot. This is partially due to the endemic nature of the disease and the relative success of chemical control with compounds such as metalaxyl. However, surveys conducted recently in the US have shown that metalaxyl-resistant isolates of the pathogen are now wide-spread. Therefore integrated disease management is becoming increasingly important due to failure of chemical control.
Recent studies conducted at North Dakota State University, to evaluate tuber susceptibility of common US potato cultivars to infection by pink rot suggested that there were some differences in susceptibility among cultivars with some being less susceptible than others. Among the white-skinned cultivars, Atlantic, LaChipper, Pike and FL 1833 were the least susceptible. Among the red-skinned cultivars Red Norland and Nordonna were the least susceptible and among the russet-skinned cultivars Ranger Russet and Russet Burbank were the least susceptible.
Chemical control options for pink rot are limited to a few compounds.
Fungicides containing metalaxyl and mefenoxam were used effectively
to control pink rot in the 1990's. Metalaxyl is the active ingredient
in Ridomil and mefenoxam in Ridomil Gold (Syngenta Corp.), Ultra Flourish
and Flouronil (Nufarm Co.). However as mentioned above, metalaxyl-resistant
isolates of P. erythroseptica are now widespread and this may
lead to failure of these chemicals to control pink rot. Studies at Michigan
State University have been largely inconclusive with respect to the
efficacy of metalaxyl/mefenoxam-based products and thus the use of these
products is no longer recommended for the control of pink rot.
The rapid development of metalaxyl and mefenoxam resistance
among populations of Phytophthora spp. as well as in P.
erythroseptica populations in some locations suggest that continuous
use of mefenoxam compounds will eventually lead to failure of chemical
control of pink rot and necessitate finding alternatives to these compounds.
An alternative to mefenoxam is a product named
Phostrol (Nufarm Americans Inc.). Phostrol shows some promise in recent
studies to control pink rot. The compound is composed of mono and dibasic
sodium, potassium and ammonium salts of phosphorous acid. Phosphorous
acid has some systemic activity and inhibits the growth of Phytophthora
spp. The exact mode of action of this compound is not fully understood.
Phostrol may be applied as a foliar or a post-harvest treatment to tubers entering storage and also has activity against
potato late blight.
Since few fungicides are available currently to control
pink rot, it is necessary to keep these fungicides effective for as
long as possible. It is important to avoid the build up of fungicide
resistant populations. This can be achieved by integrating all methods
available for management of this disease especially the use of cultural
methods and by following the guidelines laid out by the Fungicide
Resistance Action Committee