Alternaria spp. (Cotton)
|Scientific Name||Alternaria macrospora Zimm., Alternaria alternata (Fr.:Fr.) Keissl., Alternaria gossypii (Jacz.) Nisikado, K. Kimura & Miyawaki, Alternaria gossypina (Thüm) Hopkins|
|Synonyms||Alternaria longipedicellata Snowden (= A. macrospora), Macrosporium macrosporum Moray (= A. macrospora); Alternaria longipedicellata Snowden (= A. macrospora), Macrosporium macrosporum Moray (= A. macrospora)|
|Common Names||English: Alternaria leaf spot; Spanish: Pudricion por Alternaria, mancha foliar; German: Alternaria-Blattfleckenkrankheit; French: Maladie des taches brunes|
|Description||Alternaria species are ascomycetous fungi belonging to the family Pleosporaceae, order Pleosporales, class Dothideomycetes (formation of bitunicate asci in ascostroma).|
Early infections of cotyledons, leaves and bracts produce small, circular brown, gray-brown to tan spots with purple margins varying from 1 to 10 mm in diameter. They often exhibit concentric zonation and have dry, gray centres which often crack and fall out (shot-hole) when mature. The zonation is more clearly defined on the upper surface. Older spots may coalesce producing irregular dead areas. Under humid conditions, abundant sporulation of the pathogen may result in sooty black appearance of lesions. Stem lesions begin as small sunken spots which develop into a canker, the tissue splitting and cracking to cause a break. Also the glandular areas on the receptacle may be attacked resulting result in failure of boll development. Flowers and bolls may be shed; the latter become mummified and the fibre attacked.
Mycelium of A. macrospora survives on undecomposed cotton residues. The pathogen spreads with air-borne spores which may be also dispersed by splashing onto healthy tissue. Cylindrical to slightly tapering conidiophores, formed in clusters or solitarily, produce obclavate, obpyriform, sometimes ellipsoidal melanized conidia (90 - 180 x 15 - 22 µm; 20 - 65 x 9 -20 µm for A. alternata) with a narrow beak. The multi-celled, pale brown conidia typically have transverse and longitudinal oblique septae. They are produced singly or rarely in chains of two (catenulate for A. alternata). The production of conidia within leaf spots as well as the infection of healthy tissue is favored by wet weather and temperatures of about 27 °C. Gossypium barbadense (Pima cotton) is highly susceptible, while Upland cotton (G. hirsutum) is tolerant under normal weather conditions. Plants are most susceptible at the seedling stage and late in the season when the crop senescence.
Under favorable conditions - in wet seasons - disease severity decreases from lower leaves to upper leaves, unless leaves are affected by premature senescence. Susceptible cultivars can defoliate rapidly under favorable conditions, especially where the peduncle becomes infected. A. macrospora spots may also develop on bolls. Symptom development is favored by any physiological or nutritional stress e.g. heavy fruit load or premature senescence. Failure of stomatal closure associated with high relative humidity, predisposes plants to fungal infection.
Additional Crop Information
A. macrospora is largely limited to cotton and some weeds; in contrast, the host range of A. alternata is fairly wide.
Alternaria leaf spot may be confused with those of bacterial blight which are angular in shape. Alternaria leaf spot has become the most common foliar disease of cotton worldwide. A. macrospora is the major cause of cotton leaf spot; A. alternata, also often observed on cotton, is less virulent.
Useful non-chemical contribution to Integrated Weed Management
To control or minimise leaf spot problems, carryover of A. macrospora may be reduced by the incorporation of cotton residues between consecutive cotton crops and/or rotation with cereals. Avoidance of plant stress, especially potassium deficiency, delays primary infections and reduces leaf spot severity.
Generally, in cotton there are few regions and situations where Alternaria leaf spot pathogens alone are lowering yield so significantly that a specific fungicidal spray treatment is economically justified.
Spray treatments which control this disease to a limited degree have been reported earlier with preventative applications by dithiocarbamates (e.g. mancozeb) or copper products.
Fungicidal seed treatments targeting a complex of important seed- and soilborne pathogens (such as Rhizoctonia solani, Fusarium spec. and Thielaviopsis basicola) also contribute to reduce the importance of Alternaria leaf spot diseases. For this purpose in some countries (e.g. USA) broad spectrum seed treatment products for use in cotton contain (among others) either strobilurins (e.g. trifloxystrobin) or sterol biosynthesis inhibitors (e.g. triadimenol, ipconazole).
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