Modeling the effects of local climate change on crop acreage

The impacts of climate change on agriculture depend on local conditions and crops grown. For instance, warmer winter temperatures in a given area would reduce chill hours, potentially cutting yields for some crops but extending the growing season for others. Using a century of climate data and six decades of acreage data, we established quantitative economic relationships between the evolution of local climate and acreage of 12 important crops in Yolo County. We then used the historical trend in climate change to project future crop acreages in the county. Only marginal changes in acreage in 2050 were projected for tree and vine crops there, in part because chill hours, although lower, remained above critical values. Walnuts were the most vulnerable tree crop, and the projections indicated some cultivars might be marginal in years with particularly warm winters. Processing tomato acreage might increase, due to a longer growing season, and also alfalfa acreage, if water availability and other factors remain constant.

First Report of Bacterial Fruit Blotch on Melon Caused by Acidovorax citrulli in California

In July 2013, a melon plant sample (Cucumis melo cv. Saski) with disease symptoms resembling bacterial fruit blotch (BFB), was collected from a 10-acre field located in Yolo County, California, and submitted to the Plant Pest Diagnostics Center of the CDFA. Melon leaves had small (5 to 10 mm in diameter), tan to dark reddish-brown, angular lesions surrounded by yellow halos, and larger V-shaped lesions that extended from the leaf margins to the midrib. Bacterial streaming was observed at 400× magnification. The bacterium isolated from a leaf tissue wet mount formed smooth, round, cream-colored, non-fluorescent colonies on Pseudomonas F agar, was gram-negative, rod-shaped, aerobic, and oxidase-positive. The strain grew at 41°C and produced a strong hypersensitive response on tobacco (Nicotiana tabacum) 24 h after tissue infiltration. Based on a positive immunoassay test for Acidovorax citrulli (Eurofins STA Lab, Inc., Longmont, CO) and positive real-time PCR assays using species-specific primer sets, BX-L1/BX-S-R2 (1) and ZUP2436/2437 (4), the strain was identified as A. citrulli. A 360-bp fragment of the 16S ribosomal RNA gene was amplified using conventional PCR with primers WFB1 and WFB2 (3). The fragment, GenBank Accession No. KJ531595, showed 100% identity with the corresponding regions of A. citrulli (CP000512) strain AAC00-1 by BLAST query. Pathogenicity tests were performed by injecting 0.5 to 1 ml suspensions of the bacteria (106 CFU/ml) under the rind of three mature honeydew fruit (Cucumis melo var. indorus), three watermelon fruit (Citrullus lanatus cv. Sugar Baby), and into the cotyledons of ten, 10-day-old watermelon seedlings (cv. Sugar Baby). The fruit and seedlings were incubated in plastic bags at 30°C and similar treatments with sterile water served as negative controls. After 4 days, the seedlings inoculated with the suspect strain exhibited dark brown necrotic lesions with yellow halos that later coalesced, causing the cotyledons to collapse. Seven days after inoculation, the honeydew fruit exhibited dry, rotten gray cavities (4 to 6 cm in diameter) in the pericarp tissue below the rind. In contrast, the watermelon fruit had completely collapsed in a watery rot after 7 days. No symptoms were observed on the negative control fruits and seedlings treated with water. The pathogen was re-isolated from the inoculated fruit and seedlings and confirmed as A. citrulli by species-specific PCR and immunoassay as described above. The melon seed lot used to plant the field in Yolo County, CA, also tested positive for A. citrulli using species-specific real-time PCR assays (1,4). DNA fingerprinting by pulse field gel electrophoresis of Spe I-digested whole cell genomic DNA showed that all of the California A. citrulli strains were members of subgroup II (haplotype C strain) (3). These haplotypes normally occur on watermelon. BFB is a seed-transmitted disease of cucurbits and a major concern for national and global seed trade. First observed in United States commercial watermelon fields in 1989, BFB can cause economic losses up to 90% for commercial watermelon, cantaloupe, and honeydew growers (1,2). While BFB routinely occurs in the southeastern United States, this is the first official record of the disease in California. References: (1) O. Bahar et al. Plant Pathol. 57:754, 2008. (2) R. Walcott et al. J. Phytopathol. 152:277, 2004. (3) R. Walcott et al. Plant Dis. 84:470, 2000. (4) B. Woudt et al. Phytopathology 99:S143, 2009.

First Report of Cucumber green mottle mosaic virus on Melon in the United States

In July 2013, a melon (Cucumis melo var. Saski) field in Yolo County, California, was inspected as part of a phytosanitary inspection for seed production. The leaves of the plants showed mosaic, green mottle, and blotches. When plant sap was examined using a transmission electron microscope, rigid rod-shaped particles were observed. Melon plant samples were analyzed by both CDFA and USDA APHIS PPQ laboratories and tested positive using DAS-ELISA against Cucumber green mottle mosaic virus (CGMMV) (Agdia, Elkhart, IN). To confirm the presence of CGMMV, total RNA was analyzed by RT-PCR using primers CGMMV-F5370 5′-CTAATTATTCTGTCGTGGCTGCGGATGC-3′ and CGMMV-R6390 5′-CTTGCAGAATTACTGCCCATA-3′ designed by PPQ based on 21 genomic sequences of CGMMV found worldwide. The 976-bp amplicon was sequenced (GenBank Accession No. KJ453559) and BLAST analysis showed the sequence was 95% identical to MP and CP region of CGMMV isolates reported from Russia (GQ495274, FJ848666), Spain (GQ411361), and Israel (KF155231), and 92% to the isolates from China (KC852074), Korea (AF417243), India (DQ767631), and Japan (D12505). These analyses confirm the virus was CGMMV. To our knowledge, this is the first report of CGMMV in the United States. Based on our sequence data, a second set of primers (CGMMV-F5796 5′-TTGCGTTTAGTGCTTCTTATGT-3′ and CGMMV-R6237 5′-GAGGTGGTAGCCTCTGACCAGA-3′), which amplified a 440-bp amplicon from CGMMV CP region, was designed and used for testing all the subsequent field and seed samples. Thirty-seven out of 40 randomly collected Saski melon samples tested positive for CGMMV, suggesting the virus was widespread in the field. All the melon samples also tested positive for Squash mosaic virus (SqMV) using DAS-ELISA (Agdia). Therefore, the symptoms observed likely resulted from a mixed infection. The melon field affected by CGMMV was immediately adjacent to fields of cucumber (Cucumis sativus var. Marketmore 76) and watermelon (Citrullus lanatus var. Sugar Baby) crops, both for seed production with no barrier between the crops. CGMMV was also detected from symptomatic plants from both fields. Seed lots used for planting all three crops were tested and only the melon seed was positive for CGMMV, suggesting the seed as the source of infection. The sequenced 440-bp RT-PCR amplicons from CGMMV-infected cucumber and watermelon plants and melon seeds were 99% identical to the CGMMV from the field melon. A cucumber plant infected with CGMMV but not SqMV was used for mechanical inoculation at the Contained Research Facility at University of California, Davis. Inoculated cucumber, melon, and watermelon plants showed green mottle and mosaic similar to that observed in the field. CGMMV is a highly contagious virus and damage by this virus on cucurbit crops has been reported in regions where CGMMV is present (2). CGMMV was detected on cucumber grown in greenhouses in Canada with 10 to 15% yield losses reported due to this virus (1). The three cucurbit crops in Yolo County were planted in an isolated area with no other cucurbits nearby. Measures, including destroying all the cucurbit plant material, have been taken to eradicate the virus. Use of CGMMV free cucurbit seed is necessary for prevention of this disease. References: (1) K.-S. Ling et al. Plant Dis. 98:701, 2014. (2) J. Y. Yoon et al. J. Phytopathol. 156:408, 2008.