Susceptible Scions Research Articles

Conditions That Increase Latent Infection of Grafted or Non-grafted Tomatoes with Pseudomonas solanacearum.

Seedling roots of the susceptible tomato cultivar Ponderosa and the resistant rootstock cultivars LS-89 and BF-Okitsu 101, Ponderosa or LS-89 scions grafted on Ponderosa or LS-89 rootstocks were inoculated with Pseudomonas solanacearum. At 14 days after inoculation, seedlings were indexed for disease severity and harvested to recover the bacterial pathogen from stem tissues. Many of the resistant and susceptible seedlings did not show wilting symptoms but were latently infected with P. solanacearum. As plant age at the time of inoculation increased, and inoculum concentration or incubation temperature decreased, the frequency of latent infection and wilting decreased. In LS-89, plants grown at 25/20°C (day/night) showed almost no symptoms but had latent infection at 20-75%. Against five strains of P. solanacearum, the percentage of wilting and latent infection of seedlings was not different in Ponderosa or LS-89, but it was significantly different in BF-Okitsu 101. Both Ponderosa and LS-89 scions grafted on Ponderosa had nearly all wilted. However, Ponderosa scions gratfed on LS-89 wilted at nearly 60%, whereas LS-89 scions grafted on LS-89 showed no wilting. Sixty-nine percent of these symptomless LS-89 scions and 35% of the symptomless Ponderosa scions did have latent infections. These results indicated that bacterial wilt of susceptible scions grafted on resistant rootstocks was due to a movement of the pathogen from latently infected rootstocks into the scions.

The effects of rootstock and scion on tobacco mosaic virus infection in susceptible, tolerant and immune cultivars of tomato

SUMMARYReciprocal grafts were made between tomato cultivars Potentate, susceptible, and Virocross, tolerant (heterozygous for resistance gene Tm‐i) to tobacco mosaic virus (TMV) isolates of Pelham type o and between isogenic lines of cv. Craigella, susceptible and homozygous for gene Tm‐i. The grafted plants were inoculated with a type o isolate; both scion and stock inoculation were studied in the former, scion inoculation only, in the latter. With scion inoculation the virus content of a tolerant scion was greater on a susceptible stock than on a tolerant one, but that of a susceptible scion was unaffected by the type of stock: in contrast, the virus content of a tolerant stock was unaffected by the type of scion but that of a susceptible stock was less with a tolerant than with a susceptible scion. With root inoculation the virus contents of both tolerant and susceptible scions were greater on a susceptible than on a tolerant stock.With cv. Craigella the genotype Tm‐1/Tm‐1 was found to be immune to the type o isolate used, but in grafts the leaves of Tm‐1/Tm‐1 scions became tolerant to leaf inoculation when on susceptible stocks and the virus entered the stock. Tm‐1/Tm‐1 stocks became infected when attached to infected, susceptible scions and did not affect the virus content of those scions.The results indicate that a susceptible healthy stock may change the reaction of a tolerant or immune scion to infection by a strain of TMV.

Verticillium wilt of tobacco

Tobacco root and stem systems were studied to determine their relative importance in resistance to Verticillium dahliae Kleb. Three cultivars, the most susceptible and resistant of which differed widely in resistance to the disease, were grafted so that the stems and roots were united in all possible combinations. The act of grafting lowered resistance in the more susceptible cultivars. The rootstocks of the most resistant cultivar markedly reduced symptoms in the susceptible scions grafted to them, and the stems of the resistant cultivar had more resistance than the stems of the susceptible ones. A method of estimating the amount of susceptibility of the different graft combinations at different periods of growth is described. The data most closely fitted a hypothetical model in which 74 percent of the resistance in all varieties at all stages of growth was contributed by the root system, the remainder corning from the stem.

Hypersensitivity of monogenic resistant tomato scions to toxins produced in Bonny Best rootstocks invaded by Fusarium oxysporum f. lycopersici

SUMMARYPrevious experiments made by the writer showed that scions of tomato varieties that were monogenically resistant to wilt caused by Fusarium oxysporum f. lycopersici were apparently hypersensitive to toxins produced in infected Bonny Best (susceptible) rootstocks on to which they were grafted. The experiments did not prove conclusively, however, that the toxins were generated in the rootstocks, since they might have arisen from an interaction between the invading fungus and the resistant scion or by the generation of toxic materials when non‐toxic metabolites from the invaded root‐stock entered the resistant scion.In the present study it was demonstrated that the symptoms on the resistant scions arose when the fungus was still confined to the susceptible rootstocks. By double grafts, using resistant or susceptible intermediate stem pieces, it was also shown that the translocation of metabolites from invaded susceptible rootstocks through such stem pieces caused no change in their toxicity.It was concluded that resistant scions were highly sensitive to the direct action of toxins formed in invaded susceptible rootstocks, at concentrations which had no apparent effect on susceptible scions.

The ‘phloem necrosis' virus disease of tea in Ceylon

A virus disease of tea in the up‐country districts of Ceylon is described which is known as ‘phloem necrosis’ from the characteristic internal symptoms. External symptoms including leaf curl and zigzag and dwarfing of the shoots are of less constant occurrence and appear to be much influenced by climatic and growth conditions generally. Severely affected bushes are rendered entirely unproductive. The disease is shown to be transmissible by grafting to certain types of tea, nine clones of various origin having been infected so far. In others, of the type commercially favoured for replanting, no symptoms are produced. These have proved to be symptomless ‘carriers’ capable of transmitting the disease to further susceptible scions. The theoretical aspects of the results are discussed, considerations of control and the practical significance of the ‘carrying’ capacity being left for a further paper.

THE MOSAIC DISEASE OF THE HOP; GRAFTING EXPERIMENTS, II

Summary. When scions of varieties susceptible to mosaic disease were grafted on German, American, Danish, English and Seedling varieties, they developed typical mosaic disease. It appears, therefore, that “carrier” varieties are common amongst commercial varieties of hops. Certain male hops also proved to be “carriers.” In a few cases susceptible scions grafted on “carrier” stocks remained healthy. These anomalous cases require further investigation. As a rule, the “carrier” varieties show no trace whatever of mosaic disease; the leaves are of normal colour and the plant shows normal growth with high fertility. A case is recorded where one “carrier” variety (Red Vine Canada) exhibited in one season nascent symptoms of disease. The term “masked mosaic” is suggested for the appearance shown. Scions of the commercial variety Fuggles, when grafted on “carrier” plants, invariably remained healthy.

The Transmission Of “Mosaic” Disease In Hops By Means Of Grafting

Summary. In these experiments. where susceptible scions have been grafted on to “mosaic” stock, all developed “mosaic” symptoms with one exception. Circumstances were not favourable in this particular case and it. should be regarded as giving indecisive not negative evidence. From the other cases there is satisfactory proof that, the supposed “virus” can be transmitted up the growing bine. With the “carrier” experiments, all the known susceptible sciens, which were grafted on to the “carrier” (M 45) stock, exhibited “mosaic” symptoms. The variety M 45 must, therefore. be regarded as capable of transmitting the disease to susceptible hops, and as being a “carrier.” There is no evidence that the process of grafting can of itself in any way induce “mosaic” symptoms to pear in healthy susceptible hops, or in the “carrier” M 45