Studies on Pigeonpea Leaespot: Etiology and the Effect of Plant Age and Leaf Position on Susceptibility of Pigeonpea (Cajanus Cajan )to the Leafspot Pathogen (Mycovellosiella Cajani)

Abstract

In recent years incidence of Mycovellosiella leafspot damage has increased considerably especially during the wetter seasons in the pigeonpea growing areas. Regular occurrence of the disease has been noted both in the farmers fields and experimental plots in Machakos, Kitui, lower semi-arid areas of Kiambu, Murang'a and Embu districts since 1984. Results on pathogenicity test showed that the disease is incited by Mycovellosiella cajani (Henn) Rangel ex Trotter syn. Cercospora cajani (Henn = Vellosiella cajani (Rangel). Investigations on M. cajani with respect to the effects of culture media, temperature and light regimes on sporulation were conducted on the isolate from Kabete. Factors affecting conidial germination, effect of plant age and leaf position on the infection level of pigeonpea genotypes by M. cajani and the response of eight pigeonpea genotypes inoculated with M. cajani at seedling stage and at mature plant stage were studied. Mature plants were inoculated both in the glasshouse and in the field. The results showed that Mycovellosiella cajani has a fastidious nutritional requirement for growth than for sporulation. Selective subculturing produced colonies that are pure for sporulating character. Pigeonpea leaf decoction agar medium incubated at room temperature (20 - 24°C) at 24 hour light regime supported growth of many small colonies and gave abundant sporulation 10 days after plate incubation. Sporulation was noted under all environmental factors tested as long as colony growth occurred. M. cajani sporulated best in pigeonpea leaf decoction agar. The fungus showed moderate sporulation in carrot leaf decoction agar and PDA. The lowest sporulation was noted in potato carrot agar, carrot agar and pigeonpea meal agar. Highest sporulation occurred 14 days after inoculating Pigeonpea leaf decoction agar plates with conidial concentration of 2 x 10^ conidia ml“l. There was no colony growth observed in cultures incubated at 5, 10, 30 and 35°C. Colony growth and sporulation occurred at 15°, 20° and 25°C with optimum at 20°C. M. cajani sporulated both in acidic (pH 4) and alkaline (pH 10) medium. The highest and the lowest sporulation level occurred in culture plates adjusted to pH 5 and pH 10 respectively. Conidial production occurred in all the three light regime tested with the highest in 24 hours light regime 10 days after incubation. Conidia of M. cajani germinated at temperatures 15°, 20°, 25° nd 30°C with optimum germination at 25°C in concentration 2 x 10^ conidia ml"l. Conidial germination was lowest in concentration 2x10^ while there was no significant difference (P < 0.05) in germination among concentrations 2 x 10^, 5x10^ and 2x10^ conidia ml'l. Extracts of pigeonpea genotypes either suppressed or promoted conidial germination. Extract from 15 days old plants of cvs ICPL 295, KB 91/1 and 120 day old plants of cv. Katheka significantly suppressed conidial germination when compared to the control. None of the extracts tested promoted conidial germination significantly higher than the control. , M. cajani produced 1 or 2 germ tube that emerged from one conidium and penetrated host tissue passively through the stomata. Koch's postulates were verified to the effect that M. cajani was the causal agent of Mycovellosiella leafspot of pigeonpea. The fungus M. cajani attacked all the plant parts above the ground except flowers. The fungus caused severe leafspotting and defoliation of pigeonpea genotypes especially in the field. None of the pigeonpea genotypes was immune to Mycovellosiella leafspot although disease progress among the genotypes differed. In the glasshouse inoculated plant, disease progress was slow in cv. NPP 670 and high in cv. ICPL 295 while in the field, disease progress in cv. NPP 670 was much higher than among the other pigeonpea genotypes tested. Inoculum concentration affected the rate at which disease severity increased on pigeonpea leaves. The rate of increase in disease severity was higher when plants were inoculated with higher level of inoculum concentration (2 x 10^ or 2 x 10^ conidia ml'l). Fifty percent disease severity was rarely reached both in the field and the glasshouse. The highest disease severity was obtained on leaves from the lower position of the canopy. In glasshouse inoculated plants, there were no differences in disease severity between the leaves on the upper and the intermediate position of the canopy. While among the genotypes planted in the field disease severity was lower in the leaves on the upper part of the plant canopy as compared to the severity on the intermediate and lower part. Mycovellosiella leafspot was more severe at Kabete than at Kiboko. There was no significant correlation between seedling susceptibility and adult plant susceptibility.