Relations Between Carbohydrate Accumulation and Resistance of Cotton Plants to Phymatotrichum Root Rot in Dry Summers.

As observed by several investigators (3, 4, 9, 11, 12, 13, 14), phymatotrichmm root rot is notably less severe in dry than in moist summers. Increases have likewise been noted in the numbers of plants (lying from root rot after rains and irrigationis. The question has nlot been raised before, however, as to whether the low root-rot mortalitv of cottoni plants in dry seasons is due to moisture coniditionls unfavorable to the spread of the fungus, Phymatotrichumn omnivorurn (Shear) Duggar, through the soil, or to an associated change in the disease resistance of the planit. Increased resistanee of cotton plants to phymatotrichum root rot has been shown elsewhere by EATON and RIGLER (2) to follow increases in the carbohyldrate conceintrations in the root bark. In that inivestigation carbohydrate levels were altered by such means as defruiting, partial defoliation, anid adjustments in nitrogen suipply; the effects of drought oni carbohydrate levels and upon1 root rot were not considered. The data developed in this paper deal with (1) the effect of drought oii the acecumulation of carbohydrates and nitrogen in the root bark of cotton plants growing in dry and irrigated field plots, (2) with the effect of drought, in the samne experimenits, on the severity of phymatotrichum root rot, an(l (3), by miieans of laboratory measurements, with the ability of the ftungulls to grow from a moist substrate into soils with high moisture tensiolns.