Fusarium Dry Rot
Phillip Wharton and William Kirk
Department of Plant Pathology, Michigan State University
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Fusarium dry rot is one of the most important diseases of potato, affecting
tubers in storage and seed pieces after planting. Fusarium dry rot of
seed tubers can reduce crop establishment by killing developing potato
sprouts, and crop losses can be up to 25%, while more than 60% of tubers
can be infected in storage. All the commonly grown potato cultivars
in North America are susceptible to the pathogen, although some are
less susceptible than others and several breeding lines have been reported
to have a higher degree of resistance to dry rot.
The first symptoms of Fusarium dry rot are usually dark depressions
on the surface of the tuber. In large lesions, the skin becomes wrinkled
in concentric rings as the underlying dead tissue desiccates (Fig. 1).
Internal symptoms are characterized by necrotic areas shaded from light
to dark chocolate brown or black. This necrotic tissue is usually dry
(hence the name dry rot) and may develop at an injury such as a cut
or bruise. The pathogen enters the tuber, often rotting out the center
(Fig. 2). Rotted cavities are often lined with mycelia and spores of
various colors from yellow to white to pink (Figs. 2, 3).
Dry rot diagnosis may be complicated by the presence
of other tuber pathogens. Soft rot bacteria (Pectobacterium
spp.) often colonize dry rot lesions especially when tubers have been
stored under conditions of high relative humidity or tuber surfaces
are wet. Soft rot bacteria cause a wet slimy rot (Fig. 4) which can
rapidly engross the entire tuber and mask the initial dry rot symptoms.
Dry rot may also accompany late blight infection of tubers followed
by soft rot bacteria, leading to tubers with symptoms of all three diseases
Pythium leak and pink rot also cause brown to black
internal discoloration of tubers. However, these are wet rots and tubers
exude a clear fluid when squeezed.
Fusarium dry rot is caused by several fungal species in the genus Fusarium.
Fusarium sambucinum (teleomorph Giberella pulicaris)
is the most common pathogen causing dry rot of stored tubers in North
America, but other Fusarium species are also known to cause
dry rot, particularly F. solani var. coeruleum, and
F. avenaceum. In Michigan, F. sambucinum is probably
the main causal agent of dry rot, but F. solani var. coeruleum
may also be present.
Fusarium spp. are common in most soils where
potatoes are grown and can survive as resistant spores free in the soil
for very long periods of time. There are two main opportunities in the
potato crop cycle for Fusarium spp. to infect potato tubers,
in the Spring and in the Fall (Fig. 7). Fusarium sambucinum
and F. solani are commonly found on seed tubers in the spring.
Potato seed tubers are maintained at 37°F in storage which is approximately
the temperature at which F. sambucinum is dormant and consequently
there is minimal development of dry rot in storage. However, some level
of Fusarium dry rot is almost always present in commercially available
seed. During the pre-planting phase of potato production seed tubers
are warmed to about 54°F then cut into seed-pieces prior to planting.
Tubers infected with F. sambucinum are particularly susceptible
to the development of seed piece decay during this phase and in cases
of severe disease, seed pieces may rot completely before planting. Alternatively
after planting, over 50% of sprouts developing on infected tubers may
become diseased and killed outright before emergence. Damage at this
stage results in delayed or non-emergence and is usually expressed as
poor and uneven stands with weakened plants (Fig. 6). Reduction in crop
vigor then results from expenditure of seed energy used to produce secondary
or tertiary sprouts to compensate for damage to primary sprouts.
Progeny tubers may become contaminated with Fusarium
spores as they develop in the late summer and early fall. However, they
are not usually infected until harvest because the pathogen cannot cause
infection unless the the potato skin is ruptured, which rarely occurs
during the growing season. Wounds caused during harvest and handling
provide dormant spores on the tuber surface with multiple points of
entry into the tuber. Once the pathogen has penetrated the tuber skin
it begins to grow in the tuber tissue, causing dry rot lesions at the
point of entry (Fig. 2). In storage, dry rot develops most rapidly at
high relative humidity and temperatures of 60 to 70°F. Lower humidity
and temperatures retard infection and disease development. However,
dry rot may continue to develop at the lowest temperatures safe for
storage of potatoes. Young tubers appear to have some resistance to
dry rot which slows disease. Dry rot progresses noticeably faster during
the last half of the storage season.
From about 1970 - 1985, control of dry rot was primarily and effectively
achieved by the postharvest application of thiabendazole (Mertect).
However, from the late 1980's onwards, many strains of F. sambucinum
became resistant to the benzimidazole fungicides such as thiabendazole
and thiophanate-methyl, resulting in poor control of dry rot. Since
the pathogen infects through wounds, modification of tuber handling
to reduce wounding during harvest and storage, and the use of an effective
seed treatment in combination with good management practices during
the cutting process and storage of cut seed prior to planting are essential
to reducing Fusarium dry rot.
Some level of Fusarium dry rot is almost always present in commercially
available seed. Even though it is not possible at present to be 100%
sure that a seed lot is completely free of dry rot, it is sensible to
plant seed that meets established seed certification standards. Practicing
the following procedures will help prevent dry rot.
- Plant only certified seed. It is critical to purchase seed with
as little dry rot as possible, so always inspect seed carefully upon
- After careful unloading, seed should be stored at 40 to 42°F, 85
to 90% RH and be kept ventilated. Warm seed tubers to at least 50°F before handling and cutting to minimize injury and promote rapid
- Clean and disinfect seed storage facilities thoroughly before receiving
- Disinfect seed cutting and handling equipment often and make sure
cutters are sharp to ensure a smooth cut that heals easily.
- Do not store seed near a potential source of inoculum (e.g. cull
- Treat cut seed with a seed treatment to control seed piece decay
and sprout rot. See below for current recommendations for specific
- Plant seed that has a Fusarium problem in warm well drained soil
to encourage rapid sprout growth and emergence, and lessens the chance
- In the fall, harvest tubers after their skins have set and when
their core temperature is greater than 50°F.
- Monitor stored tubers often for dry rot. Grade out rotten tubers
when tubers are removed from storage for marketing.
Currently there are no commercially available biological control products
registered for the control of Fusarium dry rot. However, there are studies
underway at MSU to evaluate the use of biofungicides based on the biological
control bacteria Bacillus subtilis (Serenade, Agra Quest) and
B. pumilus (Sonata, Agra Quest), and the biocontrol fungus
Trichoderma harzianum (T-22 Planter Box, Bioworks) for control
of Fusarium dry rot on potato. These compound are being evaluated both
as seed treatments and for postharvest application in storage.
Several products have been specifically developed for control of seed-borne
potato diseases (Table 1) and offer broad-spectrum control for Fusarium
dry rot, Rhizoctonia, Silver Scurf, and to some extent Black Dot (Colletotrichum
coccodes). These include Tops MZ, Maxim MZ (and other Maxim formulations
+ Mancozeb) and Moncoat MZ. The general impact of these seed treatments
is noted in improved plant stand and crop vigor but occasionally, application
of seed treatments in combination with cold and wet soils can result
in delayed emergence. The delay is generally transient and the crop
normally compensates. The additional benefit of the inclusion of Mancozeb
is for prevention of seed-borne late blight.
Studies at MSU have shown that the most effective control
of Fusarium dry rot is achieved by the application of an effective fungicide,
such as fludioxinil (Maxim-based products), prior to planting. Treatment
of infected seed pieces with Maxim MZ at 10, 5 or 2 days before planting
significantly reduced the percentage of diseased sprouts per tuber and
significantly reduced seed piece decay in the varieties Pike and FL1879
(Fig. 8). Although it may not seem cost effective to apply seed treatments
to healthy seed, these results suggest that applying a seed treatment
up to 10 days prior to planting can provide effective control of dry
rot and increase rate of emergence, rate of canopy closure and final
plant stand (Fig. 9).
|Click images to enlarge the graphs
Although largely ineffective, thiabendazole remains registered for postharvest
use on tubers. Few alternative compounds are available for potato tuber
treatment in storage; these include chlorine-based disinfectants such
as, sodium hypochlorite, calcium hypochlorite and chlorine dioxide.
However, the use of chlorine-based products such as chlorine dioxide
is complicated because of the potentially corrosive nature of the material
and the need to activate the product in order to generate the chlorine
dioxide gas. Furthermore, limited information is available as to the
effectiveness of chlorine dioxide on potato storage pathogens and results
of some studies have suggested that chlorine dioxide does not provide
effective tuber protection against Fusarium dry rot.
Currently, studies are underway at MSU to evaluate
several of the new reduced risk fungicides, for use in postharvest applications.
These fungicides include Amistar, Zoxium, Phostrol, and Oxidate and
are all currently registered for use on potato under field conditions.