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A review of early blight of potato

2001; Volume: 7; Issue: 2 Linguagem: Inglês

2309-3811

J. E. van der Waals, Lisé Korsten, T.A.S. Aveling,

Powdery Mildew Fungal Diseases

Early blight of potatoes, causal agent Alternaria solani, causes major yield losses in most potato growing areas of the world. Leaf symptoms are characteristic dark brown to black lesions with concentric rings. In this review morphological, physiological and molecular characters of the pathogen as well as the disease cycle, epidemiology and control of the disease are discussed. The early blight situation on potatoes in South Africa is also summarised. Early blight caused by Alternaria solani Sorauer is a major foliar disease of potato (Solanum tuberosum L.). Disease symptoms are characteristic dark brown to black lesions with concentric rings, which produce a ‘target spot’ effect. Symptoms are initially observed on older, senescing leaves. The causal organism of early blight was first described by Ellis & Martin (1882) as Macrosporium solani. The first reference to the fungus as a parasite and its association with potato leaf blight was by Galloway (1891) in Australia. In the USA, Chester (1892) noted the disease on potatoes and other cultivated plants of the Solanaceae, particularly tomato (Lycopersicon esculentum Mill.) and eggplant (Solanum melongena L.). He described the symptoms and observed that the progress of early blight was slower than that of late blight, caused by Phytophthora infestans (Mont.) de Bary. He also noticed that potato plants severely affected by early blight had lower yields and produced smaller tubers. Jones (1893) was the first to suggest the name ‘early’ blight to distinguish the disease from ‘late’ blight. The name stems from the fact that early blight attacks early maturing cultivars more severely than medium or late maturing ones, whereas late blight is more severe on medium or late-maturing cultivars. A severe early blight epidemic caused shortening of the growing season due to premature death of vines, which led to smaller, immature tubers being produced. He observed that early blight was more prevalent on leaves attacked by insects and ascribed this to insect damage providing an entrance point for the fungus. He recommended the use of two to three Bordeaux sprays and later planting to control the disease. Galloway (1893) reported that early blight was widely distributed in the USA, but less abundant in the southern and far western states. He described the development of the disease and morphology of the fungus, and also suggested the use of Bordeaux sprays as control measure. Around this time, there was a controversy regarding the pathogen status of M. solani. Jones (1895) found M. solani to be pathogenic, entering the host directly or through insect wounds or stomata. However, an Alternaria species was also isolated from the lesions. Its spores resembled those of M. solani but were unable to produce characteristic early blight lesions in artificial inoculation studies. This species was thought to be responsible for ‘tip-burn’ of potato leaves. The difference between Macrosporium and Alternaria at that stage was arbitrary. (Macrosporium was thought to bear spores singly and Alternaria in chains.) Jones & Grout (1897) isolated two fungal species from potato leaves. One species was found to be the parasite causing early blight and they proposed that it be named A. solani. The other, a saprophyte found on dead or decaying leaves of many plant species, was identified as Alternaria fasciculata (Cook & Ellis) L.R. Jones & Grout (= Alternaria alternata (Fr: Fr.) Keissl.). The pathogen