Agricultural Biology
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Item PATHOLOGICAL ASPECTS OF SEED-BORNE MYCOFLORA OF PINUS CARIBAEA MORELET VAR. HONDURENSIS(1990) ADEGEYE, A. O.A survey of, the mycoflora of three seed lots of Pinus caribaea var. hondurensis was carried out using the incubation tests for the detection of seed-borne fungi on agricultural seeds. The seed-borne fungi identified were: Acromonium sp., Aspergillus flavus, A. fumigatus, A. niger, A. tamarii, Botrytis sp., Botryodiplodia theobromae, Chaetomium funicola, C. globosum, C. indicum, Cladosporium oxysporum, Dendrophoma sp., Fusarium eqruiseti, F. moniliforme var. intermedium, Gliocladium roseum, Macrophoma spp., Mucor spp., paecilomyces variotii, Penicillium citrinum, P. cyclopium, Pestalotiopsis spp., Phialophora fastigiata, Phomopsis occulta., Rhizopus sp. and Trichoderma sp. Seed-debris (pieces of cones and twigs) and stones also carried a large proportion of these pathogenic and saprophytic fungi. Using scanning electron microscope, no propagules of obligate parasitic pathogens were seen on the seed coat but spores and mycelia of some facultative pathogens were observed. Most of the seed-borne fungi were carried externally on the testa. Pinus caribaea var. hondurensis seeds inoculated with spore suspension and mycelial discs of the following fungi: Aspergillus niger, Fusarium moniliforme var. intermedium and F. eguiseti had drastic reduction in germination. F. moniliforme var. intermedium and F. eguiseti caused wilting and sudden death (damping-off) in 3 week and 3 month-old inoculated seedlings (20-60% mortality) 7 days after inoculation. Inoculated stems showed disintegration of cell-walls, parenchyma cells and fibres. Aspergillus niger had no effect on inoculated seedlings while Phialophora fastigiata had no effect on both the inoculated seeds and seedlings. Seeds which were surface sterilized for 5 minutes in 1% sodium hypochlorite and planted in autoclaved sterilized soil gave a germination percentage of 42 while seeds which were not surface sterilized but planted in sterilized soil gave 20%. Damping-off and wilting were observed in 10-12% of the seedlings which emerged from the two treatments 5-30 days after germination. Botryodiolodia theobromae, Fusarium moniliforme var. intermedium, Rhizoctonia spp. and Rhizopus spp. were isolated from the diseased seedlings. The effect of three fungicides: Benlate, Captan and Thiram in controlling the growth of the frequently occurring seed-borne fungi (Aspergillus niger. Fusarium moniliforme var. intermedium and Phialophora Fastigiata) was studied in vitro. Only Benlate at concentrations of 1 ppm and above inhibited the growth of the three fungi. Thiram inhibited the growth of Aspergillus niger and Phialophora fastigiata at 20 and 400 ppm respectively while Captan inhibited the growth of Aspergillus niger at 40 ppm.Item FACTORS INFLUENCING INFECTION, SPREAD AND SEVERITY OF TWO COLLETOTRICHUM SPECIES ON COWPEA (VIGNA UNGUICULATA (L.) WALP.) UNDER DIFFERENT CROPPING SYSTEMS(1991-04) ADEBITAN, S. A.Screenhouse and field experiments were conducted between 1987and 1989to investigate the factors affecting infection, severity and spread of two diseases, anthracnose, caused by Colletotrichum lindemuthianum, and brown blotch, caused by Colletotrichum truncatum. The following factors were studied: reaction of cowpea genotypes to infection, methods and time of inoculation in screening cowpea seedlings for resistance; plant spacing, genotype and cropping pattern in mono-cowpea and cowpea-maize intercrop; and time of cowpea introduction into cowpea-maize association. Cowpea varieties IT82E-60, IT81D-1137, and Vita-7were most susceptible to anthracnose whereas TVx 3236, IT81D-994 and IT81D-975were most resistant. Varieties IT82E-60, IT82D-699 and Ife Brown were most susceptible to brown blotch whereas TVx 3236, Vita-7, and IT8ID-1137were most resistant. Wrapping of wounded seedlings with inoculum meal of Colletotrichum at 21 days after seedling emergence enhanced optimal conditions for infection and disease development with a clear distinction between susceptible and resistant varieties. Significantly lower incidence and severity of the diseases occurred on intercropped cowpea than monocrop. Though incidence values of 25 and 43% on pods from intercropped cowpea and sole cowpea infected with C. lindemuthianum were recorded, the results indicated that the sole cowpea had higher seed yield than the intercropped. Yield averages of 458and 678kg/ha were obtained from intercropped and sole cowpea with anthracnose infection. Increasing cowpea spacing, between- and within-rows, reduced the infection and severity of both Colletotrichum species on cowpea in maize and non-maize stands. Infection rates for the pathogens were lower on cowpea intercropped than on sole and infection rates increased as plant spacing decreased. Disease infection, severity and spread significantly increased with increase in age among all the tested varieties. Infection rates were highest on IT82E-16(semi-erect type) and lowest on IT84S-2246-4(erect type). Thus, the erect variety proved more suitable than the spreading type in reducing the spread of both diseases. Cropping pattern significantly affected the incidence and severity of the fungi on cowpea decreasing generally in the order Sole > Strip > Intrarow > Doublerow > Interrow. Planting cowpea one week before maize and planting both crops simultaneously reduced the incidence and severity of the two fungi on cowpea more than when it was introduced later into maize. Infection rates were lowest on cowpea planted simultaneously with maize.Item SEASON OF HARVEST OF AMARANTHUS CRUENTUS L. IN RELATION TO SEED-BORNE MYCOFLORA AND MAJOR SHOOT DISEASES(1990-04) ADEBANJO, A.The quality of Amaranthus cruentus L. seeds were influenced both by seasons of harvest and varietal type. The wetness that accompanied harvests of the first (April-June) and second (August-October) seasons greatly-reduced the quality of seeds of cultivars. Quality reduction were in the form of seed malformation,-discolouration and mycelial growth on them. Fifteen fungal species were found to be associated with seeds of the cultivars used. There was a seasonal variability in the occurrence, distribution and the quantity of the seed-borne fungi. Least number and mean percentage seed infection were recorded for NHAC(100) while NHAC(30) and NHAC(33) carried the highest percentage inoculum on the seed testa. Three of the seed-borne fungi were highly pathogenic on seedlings of two of the cultivars while two of the fungi were seed transmitted. Six of these seed-borne mycoflora are new records on Amaranthus seeds in Nigeria. For the two year trials, significantly (p=0.05) least incidence (0%) of dieback, stem blight (8%) were recorded during the first season for NHAC(33) and NHAC(30) while 4% mean leaf blight was obtained from NHAC(100) in the third (December-February) season. Least mean percentage leaf blight was recorded for all the cultivars also by the third season. Conversely, the second season recorded the highest incidence of all these aerial symptoms of the disease caused by Choanephora cucurbitarum (Berk & Rav.) Thaxt. in all the cultivars. Hot water treatment of infected seeds of all cultivars at 60°C for 6 to 8 minutes completely disinfested them of the seed-borne fungi with consequent increase (>95%) in seedling emergence. Also soaking in or dusting of infected seeds with a mixture of Benlate-Captafol or single Captafol at 10g a.i./kg seed controlled the seed-borne fungi. Pre-harvest sprays of Benlate-Captafol mixture or Captafol at 3.3 kg/ha controlled inflorescence infection and seed-borne fungi with resultant yield increases and seedling emergence over the non-treated control.