Studies On Potato Virus Diseases Iv. Further Experiments With Potato Mosaic

Abstract

SUMMARY. Some positive evidence of the transmission of potato mosaic by the aphis Myzus persicae Sulz. is given. ttempts to induce a number of other potato‐feeding insects to transmit the disease proved negative. It has not been found possible to infect potato plants with mosaic by means of inoculation with the body juices, or salivary glands, of insects which have been bred upon mosaic potato plants. The virus of a potato mosaic inoculated into healthy tobacco plants by means of the needle produces an infectious disease–ringspot–of which the most characteristic symptom is the formation of necrotic concentric rings with a central spot. The symptoms of this disease differ in the two varieties of tobacco used–White Burley and Virginia–and the susceptibility of the former is greater. The virus of a potato mosaic transmitted to healthy tobacco by means of the aphis M. persicae produces a characteristic spot and mottle disease which is considered to be substantially the same disease as the needle‐produced ringspot. Ringspot in its various manifestations, when inoculated back to healthy potato by needle or aphis, reproduces in the potato the original mosaic with the symptoms intensified, and the infective power greatly enhanced. Although the aphis readily carries the virus of potato mosaic to tobacco, it transmits the resulting disease back to potato only with very great difficulty. This intensified mosaic can be spread from potato to potato by needle scratch with the greatest ease, but not by the aphis Myzus persicae Sulz. The spot and mottle disease produced in tobacco by aphis from mosaic potato, when inoculated into healthy potato plants by the needle, produces the same intensified form of mosaic as does the needle‐induced ringspot in tobacco, when returned to healthy potato. Tobacco ringspot can be spread from tobacco to tobacco by needle or aphis. When transmitted by aphis the symptoms differ from those produced by the needle. It is proved that the aphis Myzus persicae picks up the virus from mosaic potato with great regularity, but has so far usually failed under the writer's experimental conditions to infect healthy potato plants with the disease. The virus of tobacco ringspot is shown to increase in virulence by progressive inoculation through successive generations of tobacco plants. The increase is greatest after passage through a plant which is highly favourable to the development of the virus, such as the susceptible variety of tobacco, White Burley. This increase in virulence in some cases reaches only to a certain point, after that it tends to revert to its original intensity, but in others the virulence reached an intensity sufficient to Mil the plant. As a rule increased virulence of the virus in a given plant does not persist throughout the life of that plant. Temperatures above 80° F. mask the symptoms of the intensified mosaic in potato, but not of ringspot in tobacco. The respective symptoms show best in potato at 60° to 65° F. and in tobacco at 80° F. Needle inoculation on tobacco often produces local symptoms at the point of inoculation, but never on the potato where first symptoms appear on the young leaves. Juice from a potato plant infected with the intensified mosaic, and from a ringspot tobacco plant, was filtered through two Pasteur‐Chamberland filter candles, L 1 and L 3. The resulting filtrate originating from each plant infected healthy potato and tobacco plants with their respective diseases. This shows that the ringspot virus, and its counterpart in potato, are filter‐passing entities. Juice from known healthy potato plants when inoculated into healthy tobacco plants produced no symptoms. The virus of tobacco ringspot was inoculated into various plants with the following results: Tomato. Mottling on the leaves, rings not yet produced. Petunia. Mottling on the leaves with some distortion. Datura sp. Mottling accompanied by large necrotic areas. Solatium nigrum. No symptoms. Spinach. No symptoms. Cabbage. No symptoms. Only two plants each inoculated in (d), (e), (f).