Lettuce mosaic virus: … causes internal rib necrosis, … stimulates rusty brown discoloration … in cultivar climax

Lettuce has been the most important winter vegetable crop in the Imperial Valley for the past four years, averaging about 42,000 acres grown, and an average gross value of about $35 million. Lettuce grown for harvest in midwinter (December 15 to March 15) is planted from about September 10 to November 5. Plantings during October are largely of the cultivar Climax. During the winter harvest season of 1968–69, a disorder, later known an internal rib necrosis (IRN), occurred throughout the Imperial Valley in epidemic proportions just prior to harvest and caused serious losses in the field and in transit. Persons associated with the lettuce industry recalled the disease from past years but it had never been ranked as a major disease until 1969. No valid estimate was made of the losses from internal rib necrosis during the 1968–69 season but it was common for growers to abandon entire fields. Since then IRN has appeared sporadically in lettuce planted for midwinter harvest but not in epidemic proportions. In addition, a postharvest disorder (then called rusty rib) now called rusty brown discoloration (RBD), occurred on lettuce during storage and transit at 35°F. In February 1969, 90% of the lettuce shipments arriving in New York had RBD. The cause (or causes) of IRN and RBD has not been identified but environmental conditions (freezing temperature and rainfall), aqua ammonia damage and a synergistic reaction between beet western yellows virus and lettuce mosaic virus have been speculated as causes. Two papers presented here report the results of cooperative research by personnel of the California Agricultural Experiment Station, the California Agricultural Extension Service, and the US Department of Agricultural to determine the causes of IRN and RBD.