Xylella fastidiosa is an important pathogen of many commercial crops. Detection of X. fastidiosa is difficult due to low concentrations of the bacteria in insects and asymptomatic plant tissue, and non-uniform distribution in infected plants. A dual purpose conventional PCR and quantitative PCR (TaqManä) system was developed for the generic detection of X. fastidiosa strains. Primers HL5 and HL6, designed to amplify a unique region common to the sequenced genomes of four Xylella strains, amplified a 221 bp fragment from strains associated with PierceÕs disease of grapes, almond leaf scorch, and oleander leaf scorch disease and from DNA from an Xf strain associated with citrus variegated chlorosis. Standard curves were obtained using concentrations of Xylella ranging from 5 to 10 5 cells per reaction in water and grape extracts and 10-10 5 cells in insect DNA. Regression curves were similar, with correlation coefficients of r 2 > 0.97. In quantitative PCR, C t values ranged between 20 and 36 cycles for 5-10 5 bacterial cells per reaction. No amplicons were obtained with several non-Xf bacterial strains tested including related plant pathogenic, grape endophytic bacteria and endosymbiotic bacteria isolated from glassy-winged sharpshooters. The method was evaluated for clinical diagnosis of Xf in grapes, almonds and insect vectors. The procedure described is reliable for detection of the pathogen with a high degree of sensitivity and specificity.
Thrips exhibit different reproductive modes including thelytoky (females produced from unfertilized eggs), arrhenotoky (males produced from unfertilized eggs and females produced from fertilized eggs) and deuterotoky (females and males produced from unfertilized eggs). We investigated patterns of reproductive modes in onion thrips, Thrips tabaci Lindeman, populations and potential effects of the bacterium Wolbachia and temperature on these modes. We also examined the possibility that male-producing T. tabaci populations were resistant to the frequently used insecticides, lambda-cyhalothrin and methomyl. In New York during 2002Ð2004, T. tabaci populations were sampled from 20 onion Þelds and reproductive mode was determined by identifying sex of progeny from virgins. Half of the populations were thelytokous and half were a mix of thelytokous, arrhenotokous and deuterotokous individuals, which we refer to as a male-producing population. In two of four cases, the reproductive mode of a population from the same onion Þeld changed across years, suggesting that populations either mix or an external factor caused the change. To address the latter, we speculated that Wolbachia or high temperature mediated reproductive modes. Samples of T. tabaci representing each reproductive mode were examined for Wolbachia using diagnostic polymerase chain reaction (PCR), but it was not detected. Cytological examination of ovaries from two additional thelytokous lines also showed no evidence of Wolbachia. Similarly, high temperature did not affect sex allocation ratios in either thelytokous or male-producing populations. Male-producing T. tabaci populations were not positively correlated with resistance to lambda-cyhalothrin, or tolerance to methomyl. The role of the different reproductive modes in T. tabaci populations in onion Þelds remains unclear.
Populations of onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), were shown to differ significantly in their ability to transmit an isolate of tomato spotted wilt virus (Tospovirus: Bunyaviridae) (TSWV) collected from potato [Solanum tuberosum L. (Solanaceae)]. To gain an understanding of the basis for this variation, we generated reciprocal crosses between an efficient and an inefficient transmitting population. The resulting F1 progeny and progeny from the parental populations were tested for their ability to transmit TSWV. Our results indicate that the ability to transmit TSWV efficiently by T. tabaci is inherited as a recessive trait.
Thrips exhibit different reproductive modes including thelytoky (females produced from unfertilized eggs), arrhenotoky (males produced from unfertilized eggs and females produced from fertilized eggs) and deuterotoky (females and males produced from unfertilized eggs). We investigated patterns of reproductive modes in onion thrips, Thrips tabaci Lindeman, populations and potential effects of the bacterium Wolbachia and temperature on these modes. We also examined the possibility that male-producing T. tabaci populations were resistant to the frequently used insecticides, lambda-cyhalothrin and methomyl. In New York during 2002Ð2004, T. tabaci populations were sampled from 20 onion Þelds and reproductive mode was determined by identifying sex of progeny from virgins. Half of the populations were thelytokous and half were a mix of thelytokous, arrhenotokous and deuterotokous individuals, which we refer to as a male-producing population. In two of four cases, the reproductive mode of a population from the same onion Þeld changed across years, suggesting that populations either mix or an external factor caused the change. To address the latter, we speculated that Wolbachia or high temperature mediated reproductive modes. Samples of T. tabaci representing each reproductive mode were examined for Wolbachia using diagnostic polymerase chain reaction (PCR), but it was not detected. Cytological examination of ovaries from two additional thelytokous lines also showed no evidence of Wolbachia. Similarly, high temperature did not affect sex allocation ratios in either thelytokous or male-producing populations. Male-producing T. tabaci populations were not positively correlated with resistance to lambda-cyhalothrin, or tolerance to methomyl. The role of the different reproductive modes in T. tabaci populations in onion Þelds remains unclear.
Pierce's disease is a major threat to the California grape industry. The disease-causing bacterium Xylella fastidiosa is vectored by a number of leafhoppers including Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae). Experiments were conducted to study H. vitripennis preference, feeding, and survivorship in response to four Vitis genotypes. Plants of V. vinifera ('Chardonnay'), V. girdiana, V. candicans, and a V. rupestris x V. arizonica/candicans hybrid (D8909-17) were grown in pots in the greenhouse and transferred to laboratory conditions for experiments with field-collected H. vitripennis. A choice test without prior insect acclimation on grapes revealed that H. vitripennis selected Chardonnay over V. candicans throughout the duration of the experiment, whereas a shift in preference between D8909-17 and V. girdiana was observed over time. In a second set of choice tests, which were preceded by an acclimation on one of the four grape genotypes, significant genotype, time, and acclimation x genotype effects were observed. Chardonnay was preferred over V. candicans independent of acclimation genotype. Although H. vitripennis confined on D8909-17 excreted 1.8-fold (dry-weight corrected) the amount of insects feeding on V. candicans, differences in the rate of excreta production per insect or insect dry weight were not significant among grape genotypes. Adult mortality was greatest on V. candicans when H. vitripennis were confined in parafilm sachets for excreta collection as well as in a no-choice test. Grape genotype affected the behavior of adult H. vitripennis under controlled conditions, which may influence Pierce's disease epidemiology under field conditions.
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