The race of Sphacelotheca sorghi examined during these investigations is Race 2. Soil conditions are shown to influence race determinations with S. sorghi .
An improved method for the germination of teliospores has been determined.
Unambiguous evidence on temperature relationships with teliospore germination has not, hitherto, been available. Germination is shown to occur between 15.5O and 39.5OC, with an optimum between 28 O and 31OC. The temperature range and the shape of the response curve may vary with different collections of teliospores. In examinations of temperature and rate of germination the effect of sub- and supra-optimal temperatures is to delay the onset of germination. Growth of the germ-tube occurs over a wider temperature range than for teliospore germination, with growth occurring at 9.0 OC
Aeration has a stimulating effect on teliospore germination. Teliospores are capable of germination in advance of the maturation of the sorus and appreciable germination may be obtained from sori before the emergence of the panicle.
In aerated distilled water the density of teliospores has a marked effect on germination. Germination was virtually nil when teliospore concentrations of, or in excess of were 1:400 were employed, and maximum values were obtained when densities were lowered to 1:51,200 and 1:102,400. On the other hand when filter papers were employed, germination was not affected but the germ-tubes were reduced in length and width, and extent of branching, as the density increased.
Germination occurs between pH 2.92 and pH 7.05 in Sorenson's buffers.
The effect of the position of the teliospores on the infection of the plant has been examined and it is shown that maximum infection occurs when the teliospores are placed on the testa of ungerminated seed, or on the germinating seeds when the coleoptiles are not greater than 5 mm . in length. A trace infection only was recorded when inoculation was delayed until the emergence of the plumule through the coleoptile.
Covered kernel smut disease produces effects on the plants apart from the production of the diseased panicle. Significant reductions in height and in weight, as the result of infection, occur both in grain and in forage sorghums. In a clay soil, further, the differences between healthy and infected plants, both in height and in panicle length, was greater when additions of urea were made to the soil. ………………….