the P o w d e r y Mildew, in w h e a t var. Kenya
[~¢troducXionWhile the influence of micronutrients upon the susceptibility or resistance of a host plant to fungal infections has not been studied extensively, plant pathology texts .2 do draw attention to these relationships, as do more specialised works on micronutrients s 9. F o r s y t h ' s 6 work clearly shows t h a t rust resistance can be v e r y complex and that even non-nutrients such as D D T can increase the susceptibility of resistant varieties. As long ago as 1930, E a t o n 5 reported upon the influence of boron deficiency upon the resistance of barley to the obligate parasite Erysiphe gmm~nis. In the absence of boron, infection was extremely severe. No information regarding the action of copper appears to exist, although the addition of manganese to the soil significantly reduces the rate of infection. 3 In South Africa, E. grami~is has been reported extensively, but it is not of common occurrence, being mainly found infecting wheat and barley on irrigated lands 1.
ExperimentalAn unheated glasshouse filled with sand-culture experiments involving wheat var. K e n y a became infected with E. graminis D. C. The source of the infection could not be traced, but it is presumed that the spores were air borne, and all the wheat plants became infected eventually, the fungus being readily identifiable b y its vegetative characteristics, especially its large oidiospores.The infection was associated with v e r y changeable weather, warm days alternating with periods of sudden severe cold. The fungus spread rapidly over all the boron-deficient plants, less rapidly over copper, m o l y b d e n u m and manganese-deficient ones and slowly over the controls which received fullbalanced mineral nutrition.One group of plants was strictly comparable. These were plants of wheat grown in sand culture at the rate of three per 14-cm. pot. They were irrigated with a modified R o b b i n s solution 7, the deficient cultures lacking only this specific element. The plants were allowed to grow to full ear, which took a period of 14 weeks in the case of the control.
THE results of experiments carried out over a period of 12 months with various herbicides, applied mainly at the concentrations recommended by the manufacturers and at double these concentrations, indicate that both mature plants and seedlings of Hakea gibbosa; Cav., can be killed by spraying with 2 methyl 4 chlorophenoxyacetic acid, 2:4:5 trichlorophenoxyacetic acid and the sodium salts of both trichloroacetic acid and 2:4 dichlorophenoxyacetic acid. In most cases the addition of aromatic oils to the sprays appears to enhance the activity of the weed-killer present.In the concentrations applied the treatment is not economic when large areas are involved, but spraying with lower concentrations of 2:4 dichlorophenoxyacetic acid might prove economic if further investigated. There is little likelihood of erosion where these sprays are used as regeneration of the associated vegetation is relatively rapid and the plant remains are not uprooted from the soil.
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