STUDIES ON THE ALTERNARIA LEAF SPOT OF COTTON; GOSSYPIUM SPP., L, INDUCED BY ALTERNARIA MACROSPORA ZIMM.

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STUDIES ON THE ALTERNARIA LEAF SPOT OF COTTON; GOSSYPIUM SPP., L, INDUCED BY ALTERNARIA MACROSPORA ZIMM.

Abstract:

The incidence and severity of Alternaria leaf spot of cotton induced by Alternaria macrospora Zimm. in the northern growing zone were investigated. Field trials were conducted at Zaria, Nigeria in 1993 and 1994. Field survey carried out in 1993 revealed that there was high incidence of the disease in all the four areas visited with an average of 75.13%. Although the disease was presented in all the surveyed areas, the incidence was highest in Daudawa (80.5%). However, the incidence and severity varied considerably between the cotton plants grown at the on-set of the rain and those planted at various times in the rainy season. Symptomatological studies and pathogenicity test conducted in the screen-house in the Institute for Agricultural Research, Zaria confirmed that the Alternaria leaf spot of cotton observed was incited by A. macrospora Zimm. The leaf spot was amphigenous with irregular margin, brown at the centre with dark brown advancing border. The conidiophore was single, simple, straight and sometimes flexous with septation while the conidium was solitary, frequently catenate, mostly obclavate and rostrate with size of 127.5/xm (length), 21/μm (width) and beak which was more or less the same size with the body. The varietal resistance screening trial conducted in the field and screen house in 1994 followed the same pattern in terms of susceptibility to the disease. The ten varieties (comprising local and exotic lines) were susceptible to the disease, but the degree of susceptibility varied. In terms of breeding for resistance, Samcot-9, TXCDP37-HH-1-83, S-295 and GCA-NH-1-83 were found to be advisable and promising while CAM D-E and Tamcot Sp 215 were observed to be tolerant to the disease. Four fungicides (Benlate, Delsene M, Tecto and Dithane M4 5) were evaluated both in the laboratory {in vitro) and in the field. Delsene M, Tecto and Dithane M-45 were found to completely inhibit mycelial growth (100%) of the pathogen in all the four concentrations in the laboratory, but Benlate had a 100% inhibition at 1.2 kg a.i/ha first and 0.6kg a.i/ha with 86.3% and 67% in the lower concentrations respectively. In the field, four sprays each of the four fungicide at 7-days intervals starting with 5/7 leaf stage were found to reduce incidence and severity of the disease. However, the best result was obtained from Dithane M-45 which reduced incidence from 25.4% to 12.5% and severity from 2.67 to 1.83 at 2.5kg a.i/ha in 1994. This was followed by Delsene M, Tecto and finally Benlate. Their effect resulted in the reduction of incidence and severity of the disease with significant (P = 0.05) increased grain yield when compared with control. Dithane M-45 sprays gave 2572.2kg/ha grain cotton while control was 1388.9kg/ha. Delsene M sprays at 1.2kg a.i/ha were found to control the disease but was highly phytotoxic to the cotton under field condition. However, Delsene M at 0.6 kg a.i/ha were found to reduce disease incidence and severity without phytotoxicity. Sowing date trials conducted in 1993 and 1994 showed that July sown crop were not heavily infected. The incidence and severity of the disease was lower and the yield high when compared with the later date of planting (August), although there were no significant (P = 0.05) differences within the treatments in yield for both years. The later planted crops suffered precipitation and this resulted to premature flowering in response to favourable photoperiodism in 1993.

STUDIES ON THE ALTERNARIA LEAF SPOT OF COTTON; GOSSYPIUM SPP., L, INDUCED BY ALTERNARIA MACROSPORA ZIMM.

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