Early Blight
Phillip Wharton and William Kirk
Department of Plant Pathology, Michigan State University

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Introduction
Early blight, caused by Alternaria solani (E. & M.) Jones and Grout, is a very common disease of potato and is found in most potato growing areas. Although it occurs annually to some degree in most production areas, the timing of appearance and rate of disease progress help determine the impact on the potato crop. The disease occurs over a wide range of climatic conditions and depends in a large part on the frequency of foliage wetting from rainfall, fog, dew, or irrigation, on the nutritional status of foliage and cultivar susceptibility. While losses rarely exceed 20%, if left uncontrolled the disease can be very destructive. In Michigan and other potato growing areas, intensive fungicide treatment helps contain losses to less than 5%. In the eastern United States, foliar infection is the most critical phase of the disease, while in the West, particularly the Rocky Mountain production areas, tuber infection can be more important than foliage infection. In contrast to its name, the disease rarely develops early and usually appears on mature foliage.

Symptoms
Foliar symptoms of early blight first appear as small, irregular to circular dark brown spots on the lower (older) leaves. These may range in size from a pinpoint to an eighth of an inch in diameter (Fig. 1). As the spots enlarge, they become restricted by leaf veins and take on an angular shape. Early in the growing season, lesions on young, fully expanded succulent leaves may be larger up to half an inch in diameter and may, due to their size, be confused with late blight lesions (Fig. 2). Leaf lesions are relatively easy to identify in the field because lesion development is characterized by a series of dark concentric rings alternating with bands of light tan tissue (Fig. 3). A narrow band of chlorotic tissue often surrounds each lesion and extensive chlorosis of infected foliage develops over time. Elongated superficial brown or black lesions may also form on stems and petioles.

By the end of the growing season, the upper leaves of infected potato plants may be peppered with numerous small early blight lesions (Fig. 4) and subsequently lesions may coalesce to cover a large area of the leaf (Fig. 5a). Severely infected leaves eventually wither and die but usually remain attached to the plant (Fig. 5b). Severe infection of foliage by the early to mid-bulking period can result in smaller tubers, yield loss and lower tuber dry matter content.

Tuber symptoms of early blight include circular to irregular lesions that are slightly sunken and often surrounded by a raised purple to dark brown border. The underlying tissues are leathery to corky in texture, dry and usually dark brown (Fig. 6). These lesions reduce the quality and marketability of fresh market tubers. Tuber infection also presents a challenge to processors as tuber lesions often require additional peeling to remove the darkened lesions and underlying tissues.

Disease cycle
Early blight is caused by the fungus Alternaria solani. The dark colored spores (Fig. 7) and mycelium of the pathogen survive between growing seasons in infested plant debris and soil, in infected potato tubers and in overwintering debris of susceptible solanaceous crops and weeds including hairy nightshade (Solanum sarrachoides). Overwintering spores and mycelia of A. solani are melanized (darkly pigmented) and can withstand a wide range of environmental conditions including exposure to sunlight and repeated cycles of drying, freezing and thawing. In spring, spores (conidia) serve as primary inocula to initiate disease (Fig. 8). Plants grown in fields or adjacent to fields where potatoes were infected with early blight during the previous season are most prone to infection, since large quantities of overwintering inoculum are likely to be present from the previous crop. Initial inoculum is readily moved within and between fields, as the spores are easily carried by air currents, windblown soil particles, splashing rain and irrigation water.

Spores of A. solani (Fig. 7) are produced on potato plants and plant debris between 41 and 86°F (the optimum is 68°F). Alternating wet and dry periods with temperatures in this range favor spore production. Few spores are produced on plant tissue that is continuously wet or dry. The dissemination of inoculum follows a diurnal pattern in which the number of airborne spores increases as leaves that are wet with dew or other sources of nighttime moisture dry off, relative humidity decreases and wind speeds increase. The number of airborne spores generally peaks in mid-morning and declines in late afternoon and at night.

Spores of A. solani (Fig. 7) are produced on potato plants and plant debris between 41 and 86°F (the optimum is 68°F). Alternating wet and dry periods with temperatures in this range favor spore production. Few spores are produced on plant tissue that is continuously wet or dry. The dissemination of inoculum follows a diurnal pattern in which the number of airborne spores increases as leaves that are wet with dew or other sources of nighttime moisture dry off, relative humidity decreases and wind speeds increase. The number of airborne spores generally peaks in mid-morning and declines in late afternoon and at night.

Spores landing on leaves of susceptible plants germinate and may penetrate tissues directly through the epidermis, through stomata and or through wounds such as those caused by sand abrasion, mechanical injury or insect feeding. Free moisture (from rain, irrigation, fog or dew) and favorable temperatures (68 - 86°F) are required for spore germination and infection of plant tissues. Lesions begin to form 2 to 3 days after initial infection.

Many cycles of early blight spore production and lesion formation occur within a single growing season once primary infections are initiated. Secondary spread of the pathogen begins when spores are produced on foliar lesions and carried to neighboring leaves and plants. Early blight is largely a disease of older plant tissues and is more prevalent on senescing tissues on plants that have been subjected to stresses induced by injury, poor nutrition, insect damage, or other types of stress. Early in the growing season the disease develops first on fully expanded leaves near the soil surface and progresses slowly on juvenile tissues near the growing point. The rate of disease spread increases after flowering and can be quite rapid later in the season during the bulking period and during periods of plant stress. Early blight lesions are often found on most leaves of unprotected plants late in the growing season.

In potato tubers, germinated spores penetrate the tuber epidermis through lenticels and mechanical injuries to the skin. Tubers often become contaminated with A. solani spores during harvest.These spores may have accumulated on the soil surface or may have been dislodged from desiccated vines during harvest. Infection is most common on immature tubers and those of white- and red-skinned cultivars, since they are highly susceptible to abrasion and skinning during harvest. Course-textured soil and wet harvest conditions also favor infection. In storage, individual lesions may continue to develop but secondary spread does not occur. Infected tubers may shrivel through excessive water loss, depending on storage conditions and disease severity. Early blight lesions on tubers, unlike late blight lesions, are usually not sites of secondary infection by other decay organisms.

Monitoring and control
Effective management of this disease requires implementation of an integrated disease management approach. The disease is controlled primarily through the use of cultural practices, resistant cultivars and foliar fungicides.

Cultural control
Cultural practices, such as crop rotation, removal and burning of infected plant debris, and eradication of weed hosts helps reduce the inoculum level for subsequent plantings. Since A. solani persists in plant debris in the field from one growing season to the next, rotation with non-host crops (e.g. small grains, corn or soy bean) reduces the amount of initial inoculum available for disease initiation. Other cultural control measures may include the following.

1. Avoid irrigation in cool cloudy weather and time irrigation to allow plants time to dry before nightfall.
2. Use certified disease free seed.
3. Use tillage practices such as fall plowing that bury plant refuse.

To minimize tuber infection after harvest, tubers should be stored under conditions that promote rapid suberization as A. solani is unable to infect through intact periderm.

Resistant cultivars
Cultivars with good levels of field resistance are available, however no immunity to early blight has been found in commercial potato cultivars or in their wild parents. Highly susceptible cultivars such as Red Norland, Norchip, and Superior should be avoided in locations where early blight is prevalent and disease pressure is high. Field resistance to foliage infection is associated with plant maturity. Thus late maturing cultivars are usually more resistant than early maturing cultivars and therefore, one should avoid planting early and late cultivars in the same or adjacent fields.

Chemical control
The most common and effective method for the control of early blight is through the application of foliar fungicides (Table 1). Protectant fungicides recommended for late blight control (e.g. maneb, mancozeb, chlorothalonil, and triphenyl tin hydroxide) are also effective against early blight when applied at approximately 7-10 days intervals. Resistance to the strobilurin group of fungcides (Group 11; table 1) has been reported in south west Michigan. The geographical spread of this resistance is not known but applications of strobilurins should be made in combination with tank-mixtures of the fungicides listed above. Other products that have shown efficacy against early blight include azoxystrobin, trifloxystrobin, famoxodone, pyrethamil, fenamidone, and boscalid.

The application of foliar fungicides is not necessary in plants at the vegetative stage, when they are relatively resistant. Accordingly, spraying should commence at the first sign of disease or immediately after bloom. The frequency of subsequent sprays should be determined according to the genotype and age-related resistance of the cultivar. Protectant fungicides should be applied initially at relatively long intervals and subsequently at shorter intervals as the crop ages.

Early season applications of fungicides before secondary inoculum is produced often have minimal or no effect on the spread of the disease. Early blight can be adequately controlled by relatively few fungicide applications if the initial application is properly timed. The use of predictive models to time the first application are commonly used and for Michigan can be found on the Disease Prediction page. The first application for early blight control should be timed at 200 P days after emergence. Regular inspection of fields after plants reach 12 inches in height is recommended in order to detect early infections.