Date of Award

1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Rhizoctonia Aerial Blight (RAB) has caused severe damage on susceptible soybean cultivars throughout the southern soybean growing areas in Louisiana and the southeastern United States. During the 1983, 1984, and 1985 growing seasons, several greenhouse and field experiments were conducted to determine the influence of cultural practices (row spacings, plant populations, potassium fertilizer rates, and fungicide treatments) on RAB. A row spacing of 25 cm resulted in higher yields than row spacings of 50, 75, or 100 cm, regardless of disease rating. After the crop canopy had reached its maximum density, disease ratings were not significantly different between row spacings or plant populations. An interactive effect between increasing potassium rates and fungicide treatments had positive effects on yield through the reduction of RAB. There was a negative correlation between factor 1 (the correlation of the plant nutrients; P, Ca, Fe, and N considered as a single variable) on yield and a positive correlation with RAB disease ratings. Factor 4 (the correlation of the plant nutrient Mg and leaf area considered as a single variable) was positively correlated with yield and negatively correlated with RAB disease ratings. Viability of sclerotia buried at depths of 5, 10, and 25 cm was not significantly different than that of sclerotia maintained on the soil surface (0 cm). During a 384 period viability across all levels was reduced 60-70%. During the time sclerotia were buried in soil, cells developed thickened walls and in some instances double walls. Also intact monilioid cells were observed within cells apparently devoid of cytoplasm. Cytoplasm of sclerotia buried from 50 to 384 days contained an abundance of electron dense granules which were thought to be glycogen granules. Such modifications may be important in the formation of a protective cover and energy for the remaining viable but dormant cells within sclerotia.

Pages

108

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