Some Factors Affecting Localized and Systemic Necrotic Reactions to Virus Y in the Potato

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Abstract
Factors influencing hypersensitive and necrotic reactions to Y virus in potato seedlings, developed from common vars. like Katahdin and Snowflake, were studied mainly by means of hand inoculation in the greenhouse. This method provided more accurate and detailed information than either graft-or aphid-inoculation. The virus dosages applied by aphid-inoculation have not been sufficient to assess the hypersensitivity of seedlings accurately. The strain of Y virus used in inoculation is an important factor determining whether a seedling will give a localized reaction or not. A seedling hypersensitive to 1 strain has a 12 chance of being hypersensitive to a different strain. Seedlings hypersensitive to all tested strains of Y have been developed. Strains of Y virus differed in the virus concns. which they developed in tobacco, but this did not appear to be a factor influencing the relationship between strain of Y and the hypersensitive reaction of seedlings. In several of the seedlings, lethal and localized necrotic reactions were interchangeable, depending on plant vigor at the time of inoculation. Young, actively growing plants of hypersensitive seedlings give reactions localized in the inoculated leaves, while old plants or plants with an apparently lowered metabolism give a diffuse type of necrotic reaction which results in the inactivation of the virus in tissues distant from the inoculation sites. This is possible, because the stem tissue of such seedlings can react hypersensitively to Y virus. Histological studies confirmed these findings, and showed a correlation between the extent of necrosis in stem sections and the degree of sensitivity to Y virus.