A seed disease of wheat, rye, and barley, characterized by a more or less conspicuous brown or black discoloration of the kernel, particularly in the region of the embryo, frequently reduces the seed value and usually the sales value of affected grain. A comprehensive review of the relevant literature is given. The name "kernel smudge" is suggested for this disease. Two species of Alternaria (A. tenuis and A. Peglioni) and two of Helminthosporium (H. sativum and H. teres) are the fungi chiefly associated with it in Manitoba. The kernel smudge caused by Alternaria cannot be accurately distinguished from that caused by Helminthosporium, without a laboratory examination of the seed.Extensive trials with wheat have demonstrated that the Alternaria type of kernel smudge does not affect to any marked extent seed germination, plant emergence, intensity of root rot, and yield in the subsequent crop. It was established, however, that H. sativum does reduce germination, seedling emergence, and the yield, and at the same time causes an increased amount of root rot in this crop.Under Manitoba conditions infection of the kernels arises from air-borne spores which are usually deposited in the largest numbers at about the time the kernels are maturing. Contrary to some observations made elsewhere, the disease does not result in shrunken kernels; the largest kernels are frequently infected, and the small, shrunken ones usually free from the disease. Apparently the reason for this is that the large kernels force open their covering glumes, thus affording access to air-borne spores, whereas the glumes of small kernels remain closed and exclude such spores.The seed value of grain attacked by the virulent (H. sativum) type of kernel smudge was increased considerably when such grain was dusted with suitable organic mercury dusts (ethyl mercury phosphate or methyl mercury nitrate). Dusting with copper carbonate was relatively ineffective. The development of kernel smudge in the maturing crop was not prevented by dusting the growing plants with sulphur.