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Fusarium Dry Rot
Phillip Wharton and William Kirk
Department of Plant Pathology, Michigan State University

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Introduction
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.

Figure 1. External symptoms of Fusarium dry rot. Dark depressions form on the surface of the tuber and the skin becomes wrinkled as the underlying tissue dessicates. Figure 2. Internal dry rot symptoms.  Clumps of white to yellow mycelia line a dry necrotic cavity hollowed out from rotted tissue.

Symptoms
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).

Figure 3. Clumps of mycelium and white to pink to yellow sporulating masses form on the surface of dead skin. Figure 4. Dry rot is often followed by bacterial soft rot, which makes the tissue soft, mushy and slimy with  numerous cavities.

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 (Fig. 5).

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.

Figure 5. Dry rot (F) may also accompany late blight infection (LB) followed by soft rot bacteria (SR). Figure 6. Fusarium seed piece decay and sprout rot is usually expressed as poor and uneven stands with weakened plants.

Disease cycle
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.

Figure 7. The disease cycle of the dry rot pathogen Fusarium sambucinum.

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.

Monitoring and control
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.

Cultural control
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 receipt.
  • 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 healing.
  • Clean and disinfect seed storage facilities thoroughly before receiving seed.
  • 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 piles).
  • Treat cut seed with a seed treatment to control seed piece decay and sprout rot. See below for current recommendations for specific fungicides.
  • Plant seed that has a Fusarium problem in warm well drained soil to encourage rapid sprout growth and emergence, and lessens the chance for infection.
  • 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.

Biological control
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.

Table 1. Product name, active ingredient and FRAC resistance management grouping, type and rate of application and activity of products currently registered for control of Fusarium dry rot of potatos.

Chemical control
Seed Treatment
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.

Figure 8.  Effect of treatment with Maxim fungicide 2 days before planting on Fusarium dry rot in seed pieces.  (a) Seed pieces inoculated with Fusarium sambucinum and treated with fungicide.  (b) Seed pieces not inoculated or treated with fungicide. (c) Seed pieces inoculated with F. sambucinum but not treated with fungicide.

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).

Figure 9a. Mean percentage of dry rot affected sprouts per seed tuber from experiments carried out in 2002 (A and B) and 2003 (C and D).  Bars represent the standard error of the mean.    Treatment applied 2 days before planting (black).    Treatment applied 5 days before planting (light grey).    Treatment applied 10 days before planting (dark grey).  Treated: fungicide treated seed;  Inoc: seed inoculated but not treated; Control: seed not-inoculated, not-treated. Figure 9b. The percentage of seed piece decay in tubers from in vitro dry rot experiments carried out in 2002 (A and B) and 2003 (C and D).   Bars represent the standard error of the mean.    Treatment applied 2 days before planting (black).    Treatment applied 5 days before planting (light grey).    Treatment applied 10 days before planting (dark grey).  Treated: fungicide treated seed;  Inoc: seed inoculated but not treated; Control: seed non-inoculated, non-treated.
Click images to enlarge the graphs

Postharvest fungicides
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.

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