Losses due to early blight typically are around 20-25%; however, there have been cases of 70-80% losses. These losses can be increased when the disease is combined with other diseases like blackleg and Verticillium wilt. Despite what its name may imply, early blight typically emerges later in the season; however that is not to say that it can not occur earlier in the season.
Foliar lesions appear first on lower, older leaves and increasingly work its way up the canopy. Lesions first appear as a circular brown to black spot with concentric ridges of alternating raised and depressed necrotic tissue giving it a target-like appearance. As the lesion grows the shape becomes more angular. A yellow ring surrounds the lesion. As multiple lesions on a leaf grow they merge, leading to destructive damage to the leaf tissue. Stressed and nutrient deficient plants are more susceptible and usually show the symptoms first. During later stages of the disease, stems and petioles can show dark, oblong, fleck-like lesions.
Tuber lesions appear dark with a sunken circular to irregular shape. The texture of the diseased area is corky or leathery with gray to purplish skin developing around the lesion. Margins of decayed area can appear wet with a yellow, greenish color. During storage, as the disease progresses the tubers can become shriveled.
Primary infection begins from inoculum that survived the winter either in the soil or from plant debris or inoculum from other hosts. When the potato foliage comes in contact with the contaminated soil or plant debris it forms a lesion. This lesion then produces spores which are responsible for secondary infection. Spore formation is optimal with foliage that alternates between wet and dry conditions. Such is the case with overhead irrigation systems or frequent dew or fog. Spores that are formed can be dislodged by wind and rain and are carried to foliage and soil within that field as well as to surrounding fields.
Infection spreads to tubers through wounds during or after harvest. This occurs when a wounded tuber comes in contact with contaminated soil or plant material. Moisture on tuber surfaces and warm environments help favor infection. In order to help decrease the spread of disease store tubers in a room with forced air ventilation in order to decrease moisture that is needed for spore germination. Also storage temperatures should be below 41°F in order to slow the development of disease. The disease does not spread from tuber to tuber in storage; however it may seem that they do because it may take weeks or months for lesions to appear following infection.
The following are a list of management practices that can be done to help minimize early blight on foliage and tubers.
- Protectant fungicides - Fungicides will not completely eliminate early blight disease but can help reduce the disease severity. The best time to apply the fungicide is at the first signs of secondary spread when airborne spores first appear. Fungicide classes typically used to help control early blight include chlorothalonil, fixed copper, iprodione, mancozeb, and maneb. It is important when using fungicides to adhere to all label directions.
- Reducing plant stress - Disease development can be slowed by reducing plant stress. This is achieved by maintaining proper fertility, using high quality seed, and using proper irrigation management. If an overhead irrigation system is used allow enough time for plants to dry before night fall.
- Vine removal - Mechanical vine removal or chemical vine desiccation along with delayed harvest to promote “skin set” and reduced skinning injuries will help reduce tuber infection and decay.
- Careful handling - Proper adjustment of harvesting equipment is important to reduce the amount of tuber bruising and wounding that can lead to infection.
- Crop rotation - Rotate potatoes away from previous season infected fields to allow infested plant debris to decompose in the soil. Also, bury infected crop residue during fall or spring tillage.
- Proper Storage - Immediately after harvesting place tubers in an environment that will allow for rapid suberization and wound healing. This stage should last for about 2 weeks and should have temperatures between 50-55°F and 95% relative humidity. After this stage place tubers in cool conditions, about 41°F., with forced air ventilation in order to slow the rate of disease development.
- Stevenson, W. R., Loria, R., Franc, G. D., and Weingartner, D. P., 2001, Editor. “Compendium of Potato Diseases,” APS Press, St. Paul, Minn.
- Plissey, E. 2000. “Early blight losses often exceed 25 percent.” Valley Potato Grower.
- Preston, D. 1997. “Early blight can lead to tuber rot in storage.” Valley Potato Grower.
- Franc, G. D. 1995. “Potato Early Blight Fact Sheet” Spudman
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