RESUMEN
Sixteen Meloidogyne isolates from tomato fields in California grown with resistant cultivars were multiplied on resistant tomato in a greenhouse. Of these resistance-breaking isolates, one was identified as M. javanica, and all others as M. incognita. The reproduction of the M. javanica isolate and four M. incognita isolates on six resistant tomato cultivars and on susceptible and resistant cultivars of pepper, sweetpotato, green bean, cotton, and cowpea was evaluated and compared to an avirulent M. incognita population in greenhouse pot trials. On resistant tomato cultivars, there were minor but significant differences between the resistance-breaking Meloidogyne isolates and between the different tomato cultivars. Of the other resistant crop cultivars, pepper was resistant to all isolates and green bean to all M. incognita isolates, while cotton and cowpea allowed reproduction of one of the resistance-breaking M. incognita isolates. The resistant sweetpotato cv. Bonita behaved like resistant tomato, allowing reproduction of all five resistance-breaking isolates but not of the avirulent M. incognita. Our results showed that variability exists among resistance-breaking Meloidogyne isolates, and that isolates overcoming resistance in tomato may also be virulent on resistant sweetpotato.
RESUMEN
Field experiments were conducted over 2 years in Yuma, AZ, and Holtville, CA, to establish the relationship between soil sclerotium density of Sclerotinia sclerotiorum and the incidence of lettuce drop on different lettuce (Lactuca sativa) types under different irrigation systems, and to determine the efficacy of the biocontrol agent Coniothyrium minitans (Contans) against S. sclerotiorum on crisphead lettuce at varied sclerotium densities under different irrigation systems. There was no significant interaction of irrigation (overhead sprinkler versus furrow) with either sclerotium density or with biocontrol treatment. Lettuce drop incidence was lowest in romaine lettuce compared with crisphead or leaf lettuce at all soil sclerotium densities. There was a significant positive correlation between the sclerotial density and the percent disease incidence. Disease incidence in plots infested with 2 sclerotia/m2 of bed was not significantly higher than in control plots regardless of lettuce type. However, plots infested with 40 or 100 sclerotia/m2 of bed revealed a significantly higher disease incidence over the control in all lettuce types. A single application of Contans at planting significantly reduced the incidence of lettuce drop in all lettuce types even under high disease pressure. There were no significant differences between recommended (2.2 kg/ha) and high (4.4 kg/ha) application rates of Contans or between one or two applications of the product.
RESUMEN
Cucurbit leaf crumple virus (CuLCrV) is an emergent and potentially economically important bipartite begomovirus first identified in volunteer watermelon plants in the Imperial Valley of southern California in 1998. Field surveys indicated that CuLCrV has become established in the Imperial Valley; and field plot studies revealed that CuLCrV primarily infects cucurbits, including cantaloupe, squash, and watermelon. Full-length DNA-A and DNA-B clones of an Imperial Valley isolate of CuLCrV were obtained by polymerase chain reaction (PCR) with overlapping primers. These clones were infectious in various cucurbits and common bean (cv. Topcrop); symptoms included stunted growth and leaf crumple, curl, and chlorosis. CuLCrV was not sap-transmissible, and immunolocalization and DNA in situ hybridization studies revealed that it is phloem-limited. A CuLCrV agroinoculation system was generated, and host range studies revealed differential susceptibility in cucurbits, with squash, watermelon, cantaloupe, and honeydew melon being most to least susceptible, respectively. Germplasm screening studies identified a number of resistant cantaloupe and honeydew melon cultivars. The genome organization of this CuLCrV isolate (CuLCrV-CA) is similar to other bipartite begomoviruses, and phylogenetic analysis placed CuLCrV in the Squash leaf curl virus (SLCV) cluster of New World bipartite begomoviruses. A CuLCrV-specific PCR test was developed which allows for differentiation from other begomoviruses, including SLCV.
RESUMEN
Cotton (Gossypium hirsutum) entries were evaluated for resistance to the whitefly (Bemisia tabaci biotype B) and cotton leaf crumple (CLCr) disease during the 1999 to 2001 growing seasons in the Imperial Valley of California. Entries were evaluated for densities of whitefly adults and nymphs, and for CLCr, by visual rating and squash/dot blot hybridization analyses. Differences in whitefly densities were detected among entries, but none were highly resistant, nor was there any correlation with CLCr disease severity. Entries AP 4103 and AP 6101 had relatively low whitefly densities and were highly susceptible (high CLCr disease severity ratings and viral titers), whereas NK 2387C and DPX 1883 also had low whitefly densities but were highly resistant (no symptoms or detectable viral titers). Other entries showed moderate CLCr resistance, which was independent of whitefly density. Geminivirus DNA-A and DNA-B components were consistently detected in cotton leaves with CLCr symptoms by polymerase chain reaction (PCR) with degenerate begomovirus primers, and full-length DNA-A and DNA-B clones were obtained. Cotton seedlings inoculated with these cloned DNAs by particle bombardment developed CLCr symptoms, and progeny virus was whitefly-transmissible. Sequence analysis revealed that these clones comprised the genome of a California isolate of the bipartite begomovirus Cotton leaf crumple virus (CLCrV-CA). Thus, CLCr disease in the Imperial Valley is caused by CLCrV-CA, and cotton entries with high levels of resistance were identified.