R. H. Larson

A yellow strain of potato virus X

SummaryThe isolation of a previously undescribed yellow strain of potato virus X is reported. The strain incites bright yellow non-necrotic lesions on the inoculated leaves ofN. rustica. On certain other hosts, it also reacts differently from previously described virus X strains. The reaction of certain potato test varieties, the thermal inactivation point, the passage through stem sections of the immune variety Saco and USDA Seedling 41956, and some serological and cross protection relationships are described. It is concluded that the yellow strain is sufficiently different from other reported strains of potato virus X to warrant a separate designation.

The spread of ringspot virus X by cutting knife.

The common occurrence of a severe mosaic disease, due to infection by a ringspot type of potato virus X, in certified and foundation seed fields of Chippewa, Sebago, Pontiac, Red Warba and Katahdin under Wisconsin conditions does not seem to be adequately explained on the basis of direct foliage (3.4,8) and root (6.7) contact transmission alone. Since the spread of ringspot strains of virus X in the more recently introduced potato varieties under natural conditions in the field does occur to a great extent in some varieties, notably in Chippewa and Red Warba, it seemed desirable to test transmission by means of the cutting knife. At present there is much controversy as to the spread of potato virus X from infected to healthy tuber by means of the cutting knife (r.5), although transmission by foliage and root contact seems to be well established.

The occurrence of the brownspot strain of potato virus X

A brownspot strain of potato virus X (necrotic spot--local lesion type), isolated from naturally-infected American varieties Warba and Chippewa, has been described by Ladeburg et al. (1) and Larson et al. (2). This strain differs from the commonly-known virus X strains in that dark brown localized lesions are formed on the inoculated leaves of Nicotiana rustica L., 4 (Figure 1) and complete breakdown follows systemic infection of Datura tatula L. On N. rustica the lesions expand slowly along the veins as shown in figure 2 but the virus does not become systemic. On Nicotiana tabacum L. var. Samsun, the lesions on the inoculated leaves incited by the brownspot strain are slightly more purple than those caused by the ringspot strains. Lesions produced on the inoculated leaves of Nicotiana glutinosa L. also closely resemble those caused by the ringspot strain, being only slightly darker. The brownspot strain becomes systemic on both Samsun tobacco and N. glutinosa and can easily be overlooked in a mixture of strains. The purpose of the study was to determine the occurrence of the brownspot strain of virus X in American and European potato varieties. Stock seed tubers of the varieties tested were obtained from the Potato Introduction Station, Sturgeon Bay, Wisconsin, and from Starks Farms Inc., Rhinelander, Wisconsin. Three eye seed pieces from each of three sprouted tubers of each variety to be tested were propagated in 3-inch pots in the greenhouse, at 18-20 ~ C. under normal light conditions, until they reached a height of about 15-20 cm. Two mature leaves of Gomphrena globosa L. and not less than three mature leaves of N. rustica were inoculated with a mixture of raw juice extracted from leaves of the three potato plants of each of the varieties tested. Carborundum (600 mesh) was used as an abrasive. Brown necrotic local lesions appearing on the inoculated N. rustica leaves (Fig. 3) after 8-14 days at 24 ~ C. were isolated by means of the single lesion method (1) and transferred to other N. rustica plants. Single lesion isolations were continued when necessary and supplemented b y inoculation to D. tatula. On inoculation three sources of N. rustica 4 showed very slight differences in lesion type. The Wisconsin source of N. rustica was used throughout the remainder of this study. All experiments were carried out between March and May, 1956. The brownspot strain of virus X was isolated, after one or more single lesion transfers, from 16 out of 51 potato varieties as shown in table 1.

Crimson clover, a new host for potato virus X1

The susceptibility of members of the family Leguminosae to potato virus F has been described by Kolhner (4) . Two species. Trifol ium incarnatum L. and Trifol ium subterraneum L., were found to be systemically infected by several isolates of the virus which were recovered from inoculated plants after 6 months. During these studies, T. incarnatum was suspected of being susceptible to potato virus X. A review of the literature indicated that potato virus X is known to infect systemically only a small number of plant species outside of the family Solanaceae, i.e. 4 members of the Labiatae, 1 member of the Polygonaceae, and 4 members of the Scrophulariaceae (Table 1). The purpose of this study was to determine the extent of systemic infection by potato virus X in certain members of the Leguminosae. A preliminary report on this problem has been presented (6) .

Some common stem streaks of potato

Stem streaking is a symptom common to several disorders of the potato plant. These include infection by certain strains of virus Y (2), and by the fungi l/erticilium albo-atrum R. & B. (4), Phytophthora iufestaus Mont. (DeBary) and Rhizoctonia (6). A severe stem streaking has also been reported to result from an excess supply of manganese (1, 3). Rhizoetonia stem lesions are distinctive because of their occurrence at ground level and in relation to the newly developing underground shoots. but the stem streaks incited by Phytophthora, Verticilh;um, virus Y and excesses of manganese all affect the upper parts of the plant and are superficially quite similar in appearance at some stages. They are all quite conmIon in sonIe seed-potato producing areas so that it is important to recognize each when assessing disease incidence. This paper reports some characteristics that have been observed to distinguish the stem lesion symptoms of each of the above four common disorders.

Purple top hair sprout and low soil temperature in relation to secondary or sprout tuber formation

Frequently during the past several seasons the writer s attention has been called to nonemerging potato hills or the failure of potato seed pieces to produce an emerging plant. In many early planted potato fields observed. especially in the Racine-Kenosha area of Wisconsin, the lack of stand or nonemergence ranged from 4 to 9 per cent. When the no-sprout hills were dug, it was found that the seed pieces were finn, with well-developed, small secondary tubers growing directly from the eye (Figure 1, A) . After planting these finn field seed pieces in a warm soil in the greenhouse. weak, spindly hair sprout plants developed without the formation of sprout tubers, a A combination of factors not reported as causing sprout tubers in potatoes was observed during soil temperature studies, under controlled conditions in the greenhouse at Madison, in relation to the sprouting of hair sprout tubers collected from aster yellows (purple top) infected Sebago. I t was found that in a low temperature dark chamber at 12 ~ C., a large percentage of spindle sprout tubers produced small secondary tubers at the end of a very short sprout; whereas, in a high temperature dark chalnber at 28 ~ C., weak or spindle sprouts were produced (Figure 1, B). Likewise, when hair sprout tubers were planted in a cold soil held at about 12 ~ C., secondary sprout tubers were also formed; and in a warm soil held at about 28 ~ C., weak, hair sprout spindle plants developed as shown in figure 2. Growers planting very early in the season in cold soil are advised to move their seed stocks to medium warm storage (68 ~ to 72 ~ F.) for a period of 10 days to 2 weeks before planting to induce slight sprouting. All multiple sprout tubers and weak or hair sprout tubers can thus be eliminated, reducing to a large extent the amount that would normally be planted. The planting of normal sprouted seed pieces or number two whole tubers will reduce the yield loss due to weak or nonemerging hills.

Potato field Research Meetings Place: Rhinelander, Wisconsin and vicinity Sturgeon Bay, Wisconsin Time: August 31—September 4

The virus at the desired dilution was used for inoculating two leaves at two nodes of each vigorously growing Gomphrena ylobosa plant of the same size. The opposite two leaves at those two nodes were inoculated with the virus extract of a different concentration. The following concentrat ions were used : 1:5, 1:25, 1:125, 1:625, 1:3125. Altogether, 20 leaves were inoculated with the virus at each concentrat ion in three experiments. The results are shown in figure 2. The results obtained in these tests showed that there is a sharp decline in the activity of this strain of potato virus X when diluted from 1:5 to 1:25 and from 1:625 to 1:3125. These data give an idea regarding the infectivity of the virus at certain dilutions. By the technique employed in these tests no definite conclusions can be d rawn as regards the accurate measurement of the virus activity at different concentrations.