BIOLOGY AND EPIDEMIOLOGY OF SPIIACELOMA sp. The PATHOGEN OF COWPEA SCAB.

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BIOLOGY AND EPIDEMIOLOGY OF SPIIACELOMA sp. The PATHOGEN OF COWPEA SCAB.

ABSTRACT

Transverse sections of scab infected cowpea pods revealed the presence of conidia and conidiophores in the acervuli of Sphaceloma sp. on the pods and peduncles. The pathogen colonized the epidermis in which it produced stromatic tissue from which the conidiophores and conidia developed. Sphaceloma sp. was isolated either by plating out intact young pod or leaf lesions or by grinding these (lesions) to a paste which was subsequently dispersed on media containing cowpea extracts or on potato dextrose agar (PDA) amended with streptomycin sulphate and rose bengal. Cowpea pod agar (CpdA) and PDA amended with streptomycin sulphate (1.5 g/1) and rose bengal (0.0025 g/1 of agar) were the best media for isolation of Sphaceloma sp., when intact lesions were plated on the agar surface. The dispersal of a paste of ground lesions on agar plates was less tedious and less time consuming than plating individual lesions. Scab symptoms were induced earlier on younger cowpea plants than on older plants after plants were sprayed to run-off with a conidial suspension of Sphaceloma sp. The incubation periods for the leaf, peduncle and floral cushion were 5, 9 and 8 days, respectively, regardless of the age of plants at inoculation. However, incubation period varied from 7-10 days on the stem, depending on the age of the plant at the time of inoculation. In agar culture, the fungus grows vertically, producing very little or no radial growth. Colonies readily come off agar plates when picked with forceps; consequently the mycelial mass colonies of Sphaceloma sp. was used as measure of fungal growth. These colonies, each about 2 mm in diameter and weighing approximately 0.00lg, were incubated at temperatures of 15, 20, 26, 30, and 35 °C for 3 to 15 days on corn meal agar plus rose bengal (CMAR), malt agar plus rose bengal (MAR), and potato dextrose agar plus rose bengal (PDAR). The fungus grew at 15-26°C on all three media, but no growth occurred at 30°C. Colony weight loss was recorded on MAR and PDAR at 35°C. Among the three media, PDAR was the best for growth of Sphaceloma sp., with maximum growth occurring at 26° C. On CMAR and MAR the fungus flattened out somewhat instead of growing vertically as it does on PDAR. By the 15th day at 26°C, a gelatinous matrix coated MAR colonies. Streaking this matrix on PDAR and MAR and incubating the plates at 26°C produced many colonies of the fungus. Attempts to isolate the fungus from infected cowpea seeds were unsuccessful. In view of this, an indirect method was used to determine if scab is seed transmitted in cowpea. In the first year of the study, the incidence of scab on seedlings grown from seeds produced in a region where little cowpea is grown and scab does not occur was significantly lower than on those from seeds produced in a scab-infected field, despite the fact that seedlings were probably affected by inoculum from an adjacent, older cowpea field. In the first and second year, seed treatment with benomyl drastically reduced incidence of scab in seedlings grown from seeds produced in infected fields. The data strongly suggest seed-transmission of cowpea scab. The time taken for scab symptoms to appear on all the fields with different cowpea rotations, irrespective of mulching treatment, was 25-27 days in 1993 and 1994. In both years the incidence of scab in fields last planted with cowpea a year before the trial was the highest compared to the fields last planted with cowpea 2 or 3 years before the trial. Polyethylene mulching on cowpea plots reduced rain splashing of soil onto healthy plants and probably reduced inoculum levels of the pathogen, resulting in low disease incidence for the mulched plots. More than 75% of the first symptoms of scab in cowpea seedlings under natural inoculation occurred on the epicotyl. No scab symptoms were observed on the primary leaves. The evidence suggests that the pathogen is soil-borne either in infected trash or as perennating structures (probably chlamydospores) in the soil. Disease incidences and apparent infection rates in both 1992 and 1993 were lower in later plantings (P= 0.05). In 1992 there were higher disease incidences on all the affected plant parts than in 1993. In 1992, the grain yields obtained in the early sown crop of the variety susceptible to scab (IT 85F-1517) were the lowest (306.0 kg/ha) though there were no significant differences between these yields and those of the last planting date. In 1993, the late sown crop of IT 85F-1517 had a lower grain yield (340.0 kg/ha) which was not significantly different (P= 0.05) from that of the earliest sown crop (346.0 kg/ha). The second planting date yielded the highest grain yield (1390.0 kg/ha). The earliest sown crop of the cowpea variety resistant to scab IT 83S-720-2 in 1992 and 1993 had higher grain yields (2220.0 and 2580.0 kg/ha respectively) which were not significantly different from the grain yields obtained in the second planting dates (2067.0 and 2050.0 kg/ha). The last sown crops had the lowest yields (1670.0 and 685.0 kg/ha in 1992 and 1993, respectively). Weekly applications of benomyl or carbendazim + maneb reduced the development of scab symptoms on peduncles and gave good control of the disease on flowering cushions and pods, compared to mancozeb and untreated control plants treatments. Regardless of the plant part on which disease incidence was recorded, the areas under the disease progress curves were almost zero when benomyl or carbendazim + maneb were used to control scab. Mancozeb treated plants had significantly higher disease severity values on all aerial plant parts in comparison with plants treated with benomyl or carbendazim + maneb. However, the disease severities in plants treated with mancozeb were similar to those in the untreated (check) plants.

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