Early blight symptoms

Jacquie van der Waals
Department of Microbiology and Plant Pathology University of Pretoria This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

Early blight, which is caused by Alternaria solani Sorauer, is a major foliar disease of potatoes, causing premature defoliation. The disease is widespread in most areas where potatoes or tomatoes are grown, but is especially prevalent in the tropics and temperate zones. Early blight is found in all provinces in South Africa and is a limiting factor in production in late summer.

Field (potato plot in foreground of photo) showing early blight destruction of plants

The primary damage of early blight is due to premature defoliation of the plant. Photosynthesis rates increase and respiration rates decrease in apparently healthy tissues (21). Heavy infection early in the growing season can cause yield losses of 20 - 50% (4). Symptoms are initially observed on older, senescing leaves (13, 14, 19). Characteristic symptoms are dark brown or black lesions with concentric rings on leaves, which produce a 'target spot' effect. Enlarging lesions are often surrounded by a narrow chlorotic halo. Lesions are similar on all hosts (13).

Early blight lesion, showing typical concentric rings

The most important hosts of A. solani are tomato, potato and eggplant (13). Other hosts include horse nettle, chilli, black nightshade and non-solanaceous hosts such as wild cabbage, cucumber and zinnia (11, 14). Young, immature potato tissues and plants have a transient resistance to early blight. The effect of age on susceptibility is modified by prevailing temperature, which suggests that susceptibility is governed by physiological, rather than chronological age. Other growth or stress factors that also affect susceptibility include vigour of plant growth, soil moisture and nutrition (8, 9). High nitrogen levels, together with low phosphorous and medium to high potassium levels, decrease host susceptibility (2, 8, 9, 20). In South Africa, some potato cultivars are less susceptible than others, but none has total resistance. The four main cultivars, BP1, Buffelspoort, Up-to-date and Vanderplank, are all susceptible. Relatively high levels of resistance exist in the cultivars Mnandi and Ropedi (21).

Alternaria solani overwinters as mycelium or conidia in plant debris, soil, infected tubers or on other host plants of the same family (12, 13, 19). The primary inoculum, conidia, are produced in the spring, and are then splash or wind dispersed to the lower leaves of the plant. Spore germination is facilitated by free moisture, but can be induced by relative humidity close to saturation. Germ-tubes form appressoria, and penetrate the epidermis directly or through wounds or stomata. The minimum temperature for infection can be as low as 10°C, the maximum >35°C, and the optimum between 20°C and 30°C. Incubation periods vary greatly, depending on age and susceptibility of plants (15). Sporulation occurs between 5°C and 30°C, with the optimum around 20°C (13). A. solani is one of the few pathogens that is able to overcome a lack of prolonged humidity, by using several short wet periods (usually at night) interrupted by dry intervals during the day, otherwise known as interrupted wetting periods (3, 16). This adaptation allows A. solani to thrive equally well in areas with continuous humidity and in areas with alternating wet-dry conditions. Epidemics do not generally occur until late in the season, when the plants are most susceptible.

An Alternaria solani conidium

Various cultural practices can reduce the severity of early blight, but under situations of sufficient inoculum and environmental conditions favourable for disease, complete control will not be provided. The most effective control method is a fungicide spray programme used from early in the growing season to vine kill. It is recommended that contact fungicides be applied regularly in the early stages of the disease to prevent infection. From flowering onwards, 3-4 sprays of a systemic or contact fungicide should be applied. Proper timing of initial and subsequent fungicide applications can reduce the overall number of sprays with no significant loss of yield.

Although the A. solani pathosystem on potato and tomato has been researched extensively, various aspects remain that need to be investigated. In South Africa major gaps in our knowledge of the epidemiology and economic impact of the disease still exist. One of these is forecasting of early blight in South Africa. Although many forecasters have previously been developed for early blight on potatoes and tomatoes (5, 6, 10, 17,18), there are none that have been developed for, or are applicable to, local conditions. A forecasting model, PLANT-Plus, was developed in the Netherlands for late blight of potatoes in Europe (1) and is currently in use in South Africa (7). An early blight PLANT-Plus forecaster was recently adapted for South African conditions. The author and co-workers have conducted a study to determine the accuracy and efficiency of the predictions given by the model, and to evaluate the feasibility and cost-effectiveness of an early blight prediction to time fungicide sprays, as opposed to standard calendar-based schedules currently used in South Africa (In press). The PLANT-Plus model is a combination of empirical and three fundamental sub-models. The sub-models are the following: Climatic conditions, unprotected part of the crop and life cycle of the pathogen. The model is an internet-based system, thus providing growers with real-time information and advice. A five-day regional forecast is used to predict high-risk periods and to advise the appropriate time to apply fungicides. Results of this study indicated that the PLANT-Plus early blight-forecasting model accurately predicts infections. If spray recommendations are followed sensibly, the number of applications can be reduced and disease effectively controlled, with no negative effect on yield. PLANT-Plus holds great potential for use in the potato industry in South Africa, but it must be incorporated into the late blight forecaster to be cost-effective and have multipurpose applicability, which is the long-term goal of the project. The challenge that lies ahead is to bring about a paradigm shift in the minds of growers, to encourage the implementation of such decision support systems in South Africa.

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