Symptoms and Signs
Symptoms of soft rot include soft, wet, rotted, tan or cream-colored tissues. Rot begins on the tuber surface and progresses inward. Infected tissues are sharply delineated from healthy tissue by dark brown or black margins. Shallow necrotic spots on the tubers result from infections through lenticels. Rotting tissue is usually odorless in the early stages of decay but develops a foul odor as secondary organisms invade infected tissue. Soft rot can also infect wounded stems and roots.
Plants with blackleg are stunted and have a stiff, erect growth habit. Foliage becomes chlorotic and the leaflets tend to roll upward at the margins. Plants generally are wilted. Stems of infected plants exhibit an inky black decay. The base of the stem is often completely rotted. In relatively dry soil, only the pith may show blackening. Tuber symptoms for blackleg are similar to those of soft rot. The soft rot Pectobacterium spp. may cause wilting but affected plants lack the characteristic inky black stem decay.
Comments on the Disease
Bacteria are present on all tubers and are associated with many kinds of plants. Infections in the field are favored by high soil moisture and high temperatures. Other factors include anaerobic conditions, enlarged lenticels, and invasion by other pathogens. Bacteria enter lenticels, growth cracks, or any injury. During and after harvest, soft rot is favored by immature tubers, adverse temperatures (pulp temperatures above 70°F at harvest), mechanical damage, and free water on tuber surfaces.
Blackleg inoculum comes primarily from infected seed tubers, but it may also be spread in infested soil, contaminated irrigation water, and by insects. Blackleg is favored by cool, wet conditions at planting followed by high temperatures after emergence.
The pathogens that cause these diseases occur wherever potatoes are grown. The severity of the disease depends on seed-handling techniques, soil moisture and temperature at planting, environmental conditions, cultivar, physiological condition of the seed, amount of infection in the seed lot used, and external sources of the bacteria such as irrigation water and cull piles.
Use high quality seed. Split applications of water-soluble calcium applied at 100 to 200 pounds per acre during bulking have been shown to reduce infection and severity of soft rot. Harvest mature tubers with low pulp temperature and well-set skins, and avoid mechanical injury. Avoid excessive soil moisture before harvest to reduce lenticel infection; use clean water to wash potatoes; and avoid water films on tuber surfaces during storage. Postharvest curing and storage temperatures can be a critical component of soft rot management. Specific temperature recommendations vary depending on the level of decay evident at packing and the market destiny of the potatoes (i.e., processing, fresh market, or long-term storage); for details, consult your local farm advisor or the reference Potato Health Management, Second Edition (Johnson DA, ed. 2007. St. Paul, MN: The American Phytopathological Society).
Use pathogen-free tubers for seed. Warm seed tubers to about 55°F before planting. Provide good drainage and do not over irrigate. Eliminate cull piles and potato volunteers in rotation crops and adjacent fields.
Fungicides do not directly affect these bacterial pathogens, but seed piece treatments with fungicides can reduce invasion by other fungi and therefore reduce opportunistic infection by Plectobacterium spp. Watch for disease symptoms during routine monitoring, and keep records of your results (example form—PDF). Maintaining chlorinate recirculated wash water with a concentration of 50 to 200 ppm, depending on potato variety, is particularly important when harvesting tubers with warmer pulp temperature directly from the field.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Maxim MZ)||0.5 lb/100 lb cut seed pieces||See label||NA|
|MODE OF ACTION GROUP NAME (NUMBER1): Phenylpyrroles (12)/dithiocarbamates and relatives (M3)|
|(Serenade ASO)#||2–4 qts||4||0|
|MODE OF ACTION GROUP NAME (NUMBER1): Microbial (44)|
|COMMENTS: In-furrow application of Serenade at planting has shown to reduce incidence of soft rot.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.|
|#||Acceptable for use on organically grown produce.|