Rayda K. Krell

Potential Primary Inoculum Sources of Bean pod mottle virus in Iowa

A survey of Bean pod mottle virus (BPMV) in Iowa counties was conducted and the virus was found throughout the state. A long-term monitoring study (1989 to 2002) of the main BPMV vector, the bean leaf beetle, Cerotoma trifurcata, indicated that, in 2002, populations reached the highest abundance recorded in 14 years. Three potential sources for an early season primary inoculum source were found: (i) soybean (Glycine max) seed, (ii) overwintered bean leaf beetles, and (iii) alternate BPMV host plants. Examination of 5,804 and 8,064 soybean seedlings of two cultivars yielded 0 and 3 seedlings, respectively, infected with BPMV. In a separate test, BPMV was detected in mottled and nonmottled soybean seed. Some mottled seed did not contain BPMV, indicating that soybean seed coat mottling is an unreliable indicator for presence of the virus in seed. Of 194 naturally overwintered bean leaf beetles, only 1 transmitted BPMV to soybean. BPMV was detected serologically only in 1 alternate host, Desmodium canadense, out of 23 naturally occurring plant species collected from the field. The three inoculum sources discovered in Iowa in this study could be important primary sources when vector populations are high and indicate starting points for future epidemiological investigations.

Intraplant Sampling of Grapevines for Pierce's Disease Diagnosis

The bacterium Xylella fastidiosa Wells et al. induces Pierces disease (PD) of grapevine. This study was initiated to improve sampling protocols to identify X. fastidiosa-infected grapevines (Vitis vinifera L.) in California vineyards. Several potential PD symptoms, including leaf necrosis and chlorosis, internodal distance, petiole length and weight, and extent of cane branching, were not reliable indicators of X. fastidiosa infection. The matchstick symptom (i.e., abscised leaf blades leaving behind a dried, burnt-appearing petiole tip) was the only consistent indicator of infection in X. fastidiosa-positive grapevines. Further study revealed that leaves selected from the most basal nodes of positive canes had the highest probability of X. fastidiosa detection, with enzyme-linked immunosorbent assay. A symptom reliability index (SRI) was created to assess visual PD diagnosis by node location. The SRI values were the highest at basal node locations, but symptoms at any single node were not consistently reliable for PD diagnosis. Our results showed that PD diagnosis based on foliar symptoms was unreliable. However, taking samples from the basal portion of a cane increased the probability of X. fastidiosa detection.

Spatial Distribution of Pierce’s Disease Related to Incidence, Vineyard Characteristics, and Surrounding Land Uses

A four-year (2001 to 2004) census was conducted in 220 vineyard blocks in the San Joaquin Valley (Kern County, CA) to characterize the spatial distribution patterns of Pierce’s disease (PD) of grapevine caused by Xylella fastidiosa. Geostatistics and spatial analysis with distance indices were used to investigate relationships of PD incidence (i.e., percentage of vines with PD) and occurrence (i.e., presence or absence of PD) with vineyard characteristics and the surrounding environment using a geographic information system. The census confirmed 52 vineyard blocks with at least one X. fastidiosa-infected grapevine and PD incidence in those vineyard blocks ranged from 0.001 to 29.8%. A total of 36 of the 52 blocks had fewer than five infected vines, so spatial analysis was not valid for these blocks. For the remaining 16 blocks, the distribution of X. fastidiosa-infected vines could be categorized as no structure, random, spatial trend, and aggregation as the incidence increased. Eleven blocks showed PD distributions that were consistent with primary X. fastidiosa spread, while two blocks showed aggregations that suggested secondary or vine-to-vine spread. The distribution in one block indicated a mixture of primary and secondary spread. Two blocks had recurring aggregations of X. fastidiosa-infected vines within rows that could be the result of vector feeding on adjacent plants within rows or bacterial spread on pruning implements. Significantly higher PD occurrence was found in Flame Seedless vineyard blocks, and other vineyard characteristics such as vineyard age, pruning method, and planting density were not related to PD occurrence. Although PD occurrence was not spatially related with surrounding environment, PD incidence was spatially related to citrus. This study provides spatial information concerning the epidemiology and sampling of PD in vineyards.

Generation of Prescription Maps for Curative and Preventative Site-Specific Management of Bean Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract Site-specific pest management utilizes spatial information about pest distribution to apply control tactics only where pest density is economically high within a field. This study mapped the spatial distribution of bean leaf beetles and generated prescription maps for site-specific bean leaf beetle management. To characterize and map distributions of bean leaf beetles, geostatistics and spatial analysis with distance indices (SADIE) were used. Overwintered bean leaf beetle adults were randomly distributed within the field. Spatial distribution of the first- and the second-generation bean leaf beetles were aggregated with 76% and 93% of the variability explained by spatial dependence, respectively. This indicated that site-specific management of the first- and the second-generation bean leaf beetles could be possible. Prescription maps for site-specific bean leaf beetle management were generated based on a realistic range of three economic thresholds. This study presents guidelines for generating prescription maps for curative and preventative site-specific pest management.