TRANSMISSION STUDIES OF MAIZE STREAK DISEASE1

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Abstract
Summary.: For the foregoing investigation of the relation between the virus of streak disease and, on the one hand, the insect‐vector, Balclutha mbila Naude, and, on the other hand, the maize plant, a special technique was developed in which the inserts were manipulated and studied individually.The eggs laid by infective leafhoppers of this species produced un‐infective progeny, but the young leafhoppers might in any stage of development become infective after feeding upon a diseased maize plant. This infective power was retained during the change of skin. Experiments showed that adults might acquire the power of infection after feeding for one hour, but in lower proportion than was the case when the feeding period was of several days' duration. But even when the feeding period on a diseased plant extended through the whole course of development of the insect from the first instar to the adult stage, a proportion of the hoppers remained uninfective. Experimental study of these uninfective individuals led to the conclusion that no further periods of feeding on a diseased plant mould make them infective. Nevertheless, the progeny of such resistant females might become infective.A study of the infections produced by individual hoppers, when repeatedly transferred to fresh maize plants after short time intervals, gave no indication of a cycle of alternating infective and uninfective periods in the insect. The frequency of infection was increased by high temperatures, but was not influenced by preliminary starving of the hoppers.The power of infection was usually retained through the life of the hopper, but one definite exception was observed.Experiments showed that there occurred an incubation period of the virus in the insect, of variable duration but shortest at the higher temperatures. The minimum period observed was 6–12 hours at 30°C.Comparative studies of infection by single hoppers and groups of hoppers showed that the groups caused iufections more frequently and, on the average, in less time, but not more quickly than the quickest of the individuals feeding alone.With regard to the state of the plant at the time of infection, it was found that the frequency of infection and the incubation period of the disease in the plant were affected by the temperature and the age of the plant, but apparently not by soil moisture. Infection occurred as frequently when the hoppers fed upon the upper or lower surface of the leaf and on the young or old leaves, although infection through a young leaf might came the first symptoms to appear in an unusually short period.The passage of the virus down a leaf inoculated near its tip was not affected by certain forms of leaf mutilation. This downward movement occurred at a rate exceeding 40 cm. in two hours.From the diseased plant the virus was usually obtained by the leafhopper after feeding on the chlorotic areas but not from the green areas, except that a small proportion of hoppers usually became infective after feeding on the leaf through which infection took place, and similarly a small proportion (perhaps due to experimental error) after feeding on the green areas hang between the chlorotic areas in the fully diseased leaf.

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