Alternaria leaf blight (Alternaria alternata Fr. Keissler)
Introduction
The disease is common in southern part of India. The disease cause severe defoliation and mortality during winter months (Gunasekhar and Govindaiah, 1990-91). It manifests by the appearance of burning at leaf tip initially and spread towards the middle which later become brownish, black. Infected leaves dry up within 10 days and ultimately mulberry seedlings die after few days. Alternariabelongs to the division Deuteromycota. The spores of Alternaria alternata are multi-celled and pigmented and they are produced in chains or branching chains. The spores have a distinctive appearance that makes them easy to recognize. They are broadest near the base and taper gradually to an elongate beak.
Symptoms
Burning of leaves at the leaf tip and become brownish.
Causal organism
Alternaria alternata
Systematic position
Kingdom-Fungi
Division-Deuteromycota
Order- Hyphomycetales
Family- Dematiaceae
Genus- Alternaria
Species- alternate
Description of the pathogen
The genus Alternaria is characteristic by the large conidia. They are produced on chains, light brown to brown, ovoid or obclavate.d, muriform and divided by transverse and vertical walls, with or without beak at the tip. The hyphae and conidiophore are light brown and septate. Generally, the maximum width of spore is from 5-13 to 6-24 µm (the mean is 9 to 17 µm), and the length of spore is from 16-37 to 35-110 µm (the mean is 37 to 69 µm). The number of transverse septa is from 0 to 23. The beak length is from 4 to 90 nm. The conidiophore length is from 20 to 300 µm. Usually, The conidia formed in natural habitats are larger, have longer beaked tip, and are more uniform than those formed in common agar media. Under low temperature and dry conditions, larger conidia with shorter beak are produced. On the culture it is grey to brown, downy or cotton colony.
Pre disposing factors
Alternaria blight is worse in late summer because longer periods of dew and higher relative humidity above 95% are recorded in August and September than in June and July. At the same time, aging leaves are more susceptible. High relative humidity and dew favor the disease. The disease starts on leaves of the exposed shoots on the top of trees and eventually moves towards the lower parts of the tree.
Disease cycle
Alternaria survives winter as mycelium or spores in infected plant debris left on or in the ground, under favorable conditions in the spring, the spores quickly mature into fungal leaf spots that release more spores carried to other plants by wind and splashing rain. This cycle repeats throughout spring and summer for as long as favorable conditions persist.
Cultural control
Removing plant debris from garden plots each fall and then tilling the plot. Prune off the over matured shoots.
Chemical control
Chemical control
Apply 0.2% Indofil M45. A second spray may be done if the disease is more severe. The leaf can be used for silkworm rearing 14 days after the fungicide spray.
References
Gunasekhar V, Govindaiah, Datta RK, (1992) A new leaf blight of mulberry (Morus spp.) caused by Alternaria alternata in India. Indian J. Seric., 31(2): 131-134.
Sharma PD, (1974) A new leaf spot disease of mulberry in India. Curr. Sci., 43: 429.