A NEW MECHANICAL METHOD FOR ARTIFICIALLY TRANSMITTING SUGAR-CANE MOSAIC

Abstract

1. A new mechanical method for artificially transmitting sugar cane mosaic has been developed and used at the Insular Experiment Station of Porto Rico during the years 1925-1929. 2. The method is very simple and easy to operate and produces constant high percentage of infection. 3. Former investigators had sought to transmit sugar-cane mosaic artificially by extracting the virulent juices and injecting them into the healthy plant by means of a hypodermic needle syringe or by pricking them in after smearing the leaves. The extraction and injection of the virulent juices is always troublesome and the "virus" is quickly rendered non-infectious possibly by exposure to the air. Such methods besides being laborious and troublesome do not produce constantly high percentages of infection. 4. Bruner obtained a few infections by pricking in the virulent juices directly from the mosaic into the healthy tissues without previous extraction. The percentages of infection obtained were quite low. This was possibly due to the use of too coarse a needle. 5. The spindle of tightly rolled tender leaves has been the traditional source of the inoculum in artificial transmission experiments. Our new method (spindle-to-spindle) originally consisted in pricking in the inoculum from a mosaic into a healthy spindle held tightly together. This arrangement, though producing a constant high percentage of infection was somewhat cumbersome. 6. In its final form the leaf-slip method consists in pricking in the inoculum from a slip of mosaic cane leaf held tightly as a band around the exposed spindle of a healthy cane plant. To inoculate small cane plants, corn and other grasses, the band is held around the stem. 7. Black No. 0 and white No. 2 Asta insect pins have been used with equally good results. Ordinary pins do not produce the desired results. 8. The number of pin pricks necessary to produce infection has not been determined. Experiments in which groups of cane plants were pricked one, two and three times each, failed to develop mosaic. Ordinarily, about a hundred pin pricks are sufficient to insure infection. 9. The limiting factor seems to be exposure of the "virus" to the air. By thrusting the pin in and out rapidly in a slanting position a large number of times if there are any other requirements for infection, they will be fulfilled mechanically. The larger the number of pricks made, the larger the number of chances for infection. 10. Using a bunch of pins tied up into a brush makes the inoculation work more rapid, especially when inoculating plants with a thick spindle. Equally good results have been obtained with a bunch as with only one pin at a time. 11. Success depends on the care taken to shorten the exposure to the air, of the "virus" that is carried on the pin and on the rate at which the inoculated plant is growing. In resistant or immune cane varieties, other factors are involved, of which we as yet do not know much about. 12. When the inoculated cane plant is growing vigorously, the tissues are turgid and the sap is flowing freely. This seems to favor infection because (1) the wound made by the pin closes up on it thus excluding the air and (2) possibly because the "virus" is carried and distributed rapidly through the plant. 13. Inoculated cane plants of susceptible varieties, when growing rapidly show the first symptoms of secondary infection usually fifteen days after the inoculation. The symptoms appear and are in every respect identical with those shown by plants that have been inoculated through the agency of the insect vector of the disease, the corn aphid, Aphis maidis Fitch. The course of the disease is the same in sugar-cane plants inoculated artificially as it is in plants inoculated in nature through the agency of the insect vector.