EXPERIMENTS IN THE SUDAN GEZIRA ON CONTROL OF WILT OF DOLICHOS BEAN (DOLICHOS LABLAB) ASSOCIATED WITH ATTACK BY COCKCHAFER GRUBS (SCHIZONYCHA SP.)

Wilt of dolichos bean (Dolichos lablab) in the Sudan Gezira appeared to be due primarily to cockchafer grubs (Schizonycha sp.) in the soil attacking the hypocotyls or roots of plants up to about 6 weeks after sowing. Many wilted plants also showed symptoms of ashy stem blight (Macrophomina phaseoli), but in these the fungus was probably a secondary invader rotting roots weakened or damaged by unfavourable soil conditions or insects. Wilt was often severe in dolichos sown on land cropped to sorghum (Sorghum vulgare) in the preceding season, sorghum roots in the soil harbouring the grubs. Under moderate grub attack seed dressings containing organomercurial and gamma-benzene hexachloride (BHC) gave satisfactory protection at 0.089 % gamma-BHC/seed but were inadequate when wilt was severe. Dieldrin and aldrin at 0.044 or 0.089 %/seed gave excellent protection in all experiments, but their performance under exceptionally severe wilt conditions has yet to be tested, as also the relative efficiencies of seed treatment and soil treatment under such conditions. Fungicide-insecticide seed treatment also reduced pre-emergence rotting of germinating seeds due to grub attack. Wilt rarely occurred in dolichos planted on land fallow in the preceding season, but, even in the apparent absence of wilt, seed treatment often appreciably improved emergence, plant populations, growth and yields. These effects possibly resulted from control of root damage by soil insects, such damage reducing growth and yields but not sufficiently severely to cause wilting. Pending further investigation a powder seed dressing containing organomercurial plus 20 %; dieldrin, and applied at 1 :450 by weight to seed (about 5-6 g./acre of dieldrin), is recommended for dolichos bean in the Gezira.