[ads_dropcap]T[/ads_dropcap]he primary damage of early blight is due to premature defoliation of the plant. Photosynthesis rates increase and respiration rates decrease in apparently healthy tissues. Physiological changes are difficult to measure and evaluation of crop loss is based on the level of disease. When potato tubers become infected, the quantity and quality of marketable products is decreased and the number of secondary pathogens increases. Control of early blight has been shown to increase yield and increase profit margin to farmers.
[ads-quote-center cite=’Hafiz Muhammad Rizwan Mazhar’]Department of Plant Pathology, University of Agriculture, Faisalabad-Pakistan (E-mail: firstname.lastname@example.org)[/ads-quote-center]
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[restab title=”Pathogen” active=”active”]Alternaria solani [/restab]
[restab title=”Host”]Potato (Solanum tuberosum)[/restab][/restabs]
Symptoms of early blight occur on foliage and tubers of potatoes. Initial symptoms on leaves appear as small 1-2 mm black or brown lesions. The lesions will enlarge and are often surrounded by a yellow halo. Lesions greater than 10 mm in diameter often have dark pigmented concentric rings called “bull’s-eye” type lesion is highly characteristic of early blight. Lesions occurring on stems are often sunken and lens-shaped with a light center, and have the typical concentric rings. Beneath the surface of the lesion the tuber tissue is leathery or corky with a brown discoloration. Early blight lesions on tubers tend to be dry and are less prone to attack by secondary organisms than lesions of other tuber rots. After prolonged storage severely diseased tubers may become withered.
The causal pathogen of early blight is the fungus Alternaria solani. There is no known sexual stage and hence it is classified as a Deuteromycete. The genus Alternaria is a large and important group of pathogenic fungi, which cause a significant number of important diseases. The fungus is readily cultured on artificial media such as V8 juice where it produces a deeply pigmented gray/black hairy colony. The mycelium is haploid and septate, becoming darkly pigmented with age. Sporulation in culture can be stimulated by exposure to fluorescent light. The asexual conidia are borne singly or in a chain of two on distinct conidiophores. The beaked conidia normally possess 9–11 transverse septae. Morphological and pathogenic variability among isolates of A. solani has given rise to claims of the existence of races, although this remains unproven.
Disease Cycle and Epidemiology
Alternaria solani overwinters primarily on infected crop debris. The dark pigmentation of the mycelium increases resistance to lysis which extends the survival time in the soil to several years. Warm, humid (24-29°C/ 75-84°F) environmental conditions are conducive to infection. In the presence of free moisture and at an optimum of 28-30°C (82-86°F), conidia will germinate in approximately 40 minutes. Germ tubes penetrate the leaf epidermis directly or enter through stomata. Infection of potato tubers usually occurs through wounds in the tuber skin inflicted during harvest. Wet conditions at harvest provide a favorable environment for spore germination as well as causing swollen lenticels on the tubers which are easily invaded.
Early blight is principally a disease of aging plant tissue. Lesions generally appear quickly under warm, moist conditions on older foliage and are usually visible within 5-7 days after infection.
A long wet period is required for sporulation but it can also occur under conditions of alternating wet and dry periods. Secondary spread of the disease results from conidia being dispersed mainly be wind and occasionally by splashing rain or overhead irrigation. Early blight is considered polycyclic with repeating cycles of new infection. This is the period when the disease has the potential to spread rapidly and build up to damaging levels in the crop.
In many cases employing sound cultural practices that maintain potato plants in good health will keep early blight losses below economic levels. Because the pathogen over winters on infected crop debris, in-field sanitation procedures that reduce initial inoculum in subsequent crops are beneficial. Consideration should be given to removing potentially infected material. Optimal tuber maturity is the most important factor for control of tuber infection. Tuber infection can be reduced by careful handling during harvest to minimize wounding as well as avoiding harvesting during wet conditions. Tubers should be stored at 50 to 55 ºF, at high relative humidity and with plenty of aeration to promote wound healing which will reduce the amount and severity of tuber infections that develop in storage.
Fungicides with protectant and curative properties are registered for use against early blight on potato. The cheaper protectant fungicides such as mancozeb and chlorothalonil are the foundation of most early blight management programs. These fungicides must be reapplied every 7-10 days to provide protection of new growth as well as to counter the effects of weathering which progressively removes the chemical from the leaf surface. Fungicides which are registered for Alternaria control in potato include azoxystrobin, pyraclostrobin, trifloxystrobin, fenamidone and famoxidone. They are weakly curative and use rates are considerably lower than the traditional protectant products, although cost per acre is typically higher.
In modern agriculture the potato are relatively recently adopted crops. Wide scale cultivation gained prominence largely in the 19th century some 300 years after their introduction into Europe from the Andean region of South America. The potato is a dietary staple in nearly all temperate countries with annual production worldwide in 2017 of around 381 million metric tones (FAOSTAT, 2017). Early blight is distributed worldwide and essentially occurs wherever potatoes are grown. Crop losses due to early blight in unsprayed fields vary enormously from 5 –78%. Best estimates suggest annual expenditure globally on fungicides for control of Alternaria spp. is around $45 million in potatoes.
Social Impact and Economic Importance of Blight
The possible economic and social impact of this disease is best demonstrated by the significant role it played in the Irish Famine in the middle of the 19th century. Irish farmers depended on the potato for their primary source of food. As a result of the famine, millions of Irish died or move to another country.
Late blight is a serious economic threat in the vast majority of potato production systems. In locations where disease pressure is high, a susceptible potato variety may require fungicide applications every 3–5 days. In spite of the obvious destructive potential of late blight, it is extremely difficult to measure losses due to this plant disease because other factors simultaneously affect yield.