In 1988, Arizona's early- season lettuce crop was plagued by disease and insect problems, both intensified by unseasonably high temperatures. In the western Arizona production area, an epidemic of lettuce infectious yellows (LIY) resulted in serious economic losses to growers. The yellows disease is incited by the LIY virus (LIYV), a plant virus transmitted by the sweet potato whitefly [Bemisia tabaci (Gene.)]. Disease symptoms in lettuce include stunted growth, rolling yellowing and /or reddening of infected leaves; necrotic lesions appear at or near the leaf margins at latter stages of the disease. LIYV has a wide host range which increases the difficulty of isolating lettuce fields from LIYV infected or whitefly-infested fields; also, whiteflies are resistant to insecticides. Therefore, host-plant resistance appears to be the most promising means of reducing losses due to this disease. To initiate a breeding program, commercial lettuce cultivars and breeding lines (Lactuca sativa L.), and related, cross-breeding wild lettuce species (L. serriola L. and L. saligna L.) were screened for resistance to LIYV in the western Arizona production area using natural inoculation by residence whiteflies.

Lettuce infectious yellows virus (LIYV), a recently recognized plant virus, causes dramatic yellowing symptoms and severe diseases in a wide range of vegetable crops in Arizona, adjacent southwestern states and Mexico. Until now, the only available diagnostic method was a time-consuming bioassay that used the insect vector to transmit the virus, with subsequent manipulation of indicator plants. A rapid, sensitive diagnostic technique (termed an indirect enzyme-linked immunoassay, called indirect ELISA) system was developed to detect lettuce infectious yellows virus (LIYV) in infected plant material. A virus specific antibody was made to viral capsid protein which was purified by polyacrylamide gel electrophoresis. The indirect ELISA system was optimized and used to detect viral antigen in greenhouse-inoculated melons. The system was subsequently adapted to detect LIYV in symptomatic and asymptomatic weed and cultivated plant species collected from infected fields near Yuma and in central Arizona. The indirect ELISA system described here allows for the detection of approximately 100 ng of virus per well. The LIYV was detectable in symptomatic (but not in asymptomatic) leaves of melon plants infected with the virus. In contrast, the virus could be detected in both symptomatic and symptomless cheeseweed plants collected in the field. The optical density readings for infected weed species were generally lower than those for cultivated species, such as melons, lettuce, and spinach, suggesting that there is less virus in the weed hosts tested than in infected, cultivated hosts.

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Most lettuce (Lactuca sativa L) seed fails to germinate at high temperatures. This phenomenon thermodormancy, is common in desert regions where 87% of all lettuce is grown in the U.S.A. A study was conducted using a non-thermodormant plant introduction, PI 251245, and two highly thermodormant Dutch butterhead cultivars, 'Dabora' and 'Severa'. Reciprocal crosses were made and germination trials conducted to observe how maternal and paternal influence and seed color contribute to thermodormancy. At 25 °C, germination was 100% for the three parents and the reciprocal F1 hybrids. Germination differences occurred at both 30° and 35 °C among the parents, with P1251245 with 100% germination and Dabora and Severa with less than 10% germination at both temperatures. Segregating F3 and F4 populations from Dabora x PI 251245 were investigated further. Genetic variation found between families suggests that breeding lettuce for improved thermotolerance may be possible. Seed color did not influence thermodormancy.

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