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In crosses of lettuce (Lactuca sativa L.) between parents producing a mild or susceptible reaction to lettuce mosaic virus, a single gene segregated. The heterozygote reacted in an intermediate manner. In crosses between mild- reacting and resistant parents, the mild reaction gene and the resistant gene segregated independently. The resistant and mild alleles together produced a new phenotype that is usually symptomless. The gene symbol proposed is Mi'Mi, where Mi' gives the mild phenotype. Breeding is in progress to combine the mild and resistant traits in new lettuce cultivars. All studies were conducted in a greenhouse. Seed was sown in flats or plastic pots in a sand-soil mixture. Plants were inoculated in the seed containers at the three- to four-leaf stage. The virus used was originally collected from a single plant in a lettuce field in the Salinas Valley in 1958. It has been maintained by plant to plant transfer and is available. No detectable change has been noted. The vector was the green peach aphid, cultured on a long white radish, transferred to LMV infected lettuce plants, and then to healthy seedlings. Symptoms were read at 12 to 14 d and 21 d or more, depending upon the reaction expected. The numbers of plants showing each type of reaction were recorded and the data were analyzed by chi-square. An additional experiment was carried out to clarify the results obtained in the inheritance studies. Four genotypes with different levels of resistance were compared in a nonreplicated experiment for percent infection and days to first symptom expression. Results The F1 of BAG x 'Salinas' showed severe symptoms. Two F2 populations were grown. The first (93-713) was observed only for mild versus severe symptom expression (Table 2). However, intermediate symptom expression could often be distinguished from mild and severe, and the second population (94-431) was observed for all three phenotypes (Table 2). In this and other populations, plants with intermediate expression were sometimes difficult to separate from those classified as either mild or severe. Therefore two classes have been combined, usually intermediate with mild, occasionally intermediate with severe, depending upon overall severity of symptom expression, which in turn depended on light and temperature conditions at the time of the experiment. Segregation in population 93-713 approached 3 mild : 1 severe and in population 94-431 approached 1 mild : 2 Lettuce mosaic has been a serious worldwide disease problem in lettuce (Lactuca sativa L.), and a few other leafy vegetable species, for a long time. It was first identified as a virus (LMV) disease by Jagger (1921). It can be seriously debilitating, causing losses of up to 100%, especially in production districts like the Salinas Valley of California, where the disease can be transmitted from older to younger crops throughout most or all of a long growing season, primarily by the green peach aphid (Myzus persicae Sulz.). Two methods of control have been developed: 1) use of resistant cultivars and 2) seed indexing and use of seed stocks virtually free of the virus (Grogan, 1980; Ryder, 1975). Both methods of control have been effective. This paper describes a gene controlling the type of reaction to lettuce mosaic virus (LMV) in the lettuce plant. A susceptible plant reacts systemically, manifesting vein clearing, mottling, leaf recurving, leaf margin frilling, distortion, stunting, and occasional necrosis. Resistance is also manifested systemically with one or a few small chlorotic areas on veins or vein junctions and considerably less stunting. The term resistance is used in a generic sense; a more precise and descriptive term is resistance to virus increase and spread (Bjorling, 1966). The mild reaction discussed in this paper is also systemic, but with milder effects, such as less obvious mottling, less leaf distortion, and less stunting, as compared to the susceptible reaction.

A Mild Systemic Reaction to Lettuce Mosaic Virus in Lettuce: Inheritance and Interaction with an Allele for Resistance