First Report of Leaf Rust of Blueberry Caused by Pucciniastrum vaccinii in Hawaii

Blueberries (Vaccinium corymbosum L.) are a potential high-value, niche market crop for Hawaii. In May of 2007, rust-like symptoms were observed on multiple blueberry plants in a private nursery in Waimea, HI. In September of 2007, a similar leaf rust was observed on one bush of V. corymbosum cv. Sharpblue in the corner of a 36.6 × 9.1-m experimental plot at Mealani Research Station in Waimea. Within a month, rust was observed throughout the plot on 'Biloxi', 'Emerald', 'Jewel', 'Misty', 'Sapphire', and 'Sharpblue'. Preliminary field observations suggest that 'Sharpblue' and 'Sapphire' are highly susceptible to the rust and 'Biloxi' shows some tolerance. Leaf lesions began as approximately 1-mm² chlorotic flecks that expanded and developed into reddish brown, necrotic spots with a chlorotic halo. New lesions and uredinia kept appearing over the course of 4 months. Defoliation occurred on plants where infection was severe. Yellowish orange pustules containing urediniospores first appeared on the abaxial side of older leaves and later appeared on new leaves. Urediniospores were elliptical to obovate (19.4 to 24.8 × 15.2 to 19.8 μm) with a thick, slightly roughened wall and a well-developed pore. Urediniospore morphology and dimensions were consistent with the description of Pucciniastrum vaccinii (G. Wint.) (1). A pathogenicity test was conducted with two 18-month-old 'Sharpblue' plants. Fully expanded leaves were sprayed with freshly collected urediniospores (3.8 × 10⁵ spores per ml) suspended in a 0.05% solution of Tween 20 in water. The control plant was sprayed with sterile distilled water (SDW). Plants were covered with plastic bags for 48 h and held in a growth chamber at 20 to 22°C under continuous fluorescent lighting. The plastic bags were then removed and the plants were maintained in the growth chamber. Yellowish orange pustules that were identical to the original symptoms developed on 100% of inoculated leaves after 10 days. The plant inoculated with SDW remained symptomless. While leaf rust caused by P. vaccinii has been reported on Ohelo berry (V. reticulatum) (2), it has not been reported on V. corymbosum in Hawaii. To our knowledge, this is the first report of P. vaccinii on blueberry plants in Hawaii. This rust disease may pose a threat to the potential blueberry industry in Hawaii. References: (1) P. R. Bristow and A. W. Stretch. Page 20 in: Compendium of Blueberry and Cranberry Diseases. F. L. Caruso and D. C. Ramsdell, eds. The American Phytopathological Society, St. Paul, MN, 1995. (2) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory. Online publication. ARS, USDA, 2008.

Diversity and Mealybug Transmissibility of Ampeloviruses in Pineapple

Mealybug wilt of pineapple (MWP) is one of the most destructive diseases of pineapple (Ananas comosus) worldwide. At least one Ampelovirus species, Pineapple mealybug wilt associated virus-2 (PMWaV-2), and mealybug feeding are involved in the etiology of MWP. A previously undescribed Ampelovirus sharing highest homology with PMWaV-1 and a putative deletion mutant sharing highest homology with PMWaV-2 were detected with reverse transcription-polymerase chain reaction (RT-PCR) assays using degenerate primers. Results were verified with additional sequence information and by immunosorbent electron microscopy. Sequence homology between the virus tentatively designated PMWaV-3, and PMWaV-1 and PMWaV-2, decreases toward the N-terminal across the HSP70 homolog, small hydrophobic protein, and RNA-dependent RNA polymerase open reading frames (ORF). Putative PMWaV-3 could not be detected with four different monoclonal antibodies specific for PMWaV-1 and PMWaV-2. The potential deletion mutant spanning the N-terminal of the HSP70 region was obtained from a pineapple accession from Zaire maintained at the USDA-ARS National Clonal Germplasm Repository in Hawaii. Putative PMWaV-3, like PMWaV-1 and PMWaV-2, is transmissible separately or in combination with other PMWaVs by Dysmicoccus brevipes and D. neobrevipes mealybugs. Plants infected with PMWaV-3 that were continuously exposed to mealybugs did not develop symptoms of MWP in the absence of PMWaV-2. Specific RT-PCR assays were developed for detection of putative PMWaV-3 and the deletion mutant.

Molecular Diversity of Ralstonia solanacearum Isolated from Ginger in Hawaii

ABSTRACT The genetic diversity of Ralstonia solanacearum strains isolated from ginger (Zingiber officinale) growing on the island of Hawaii was determined by analysis of amplified fragment length polymorphisms (AFLPs). Initially 28 strains of R. solanacearum collected from five host plant species worldwide were analyzed by AFLP. A second analysis was conducted on 55 R. solanacearum strains collected from three ginger farms along the Hamakua Coast of Hawaii, the principle area of ginger cultivation in the state. From the initial analysis, R. solanacearum strains from ginger in Hawaii showed a high degree of similarity at 0.853. In contrast, the average genetic similarity between R. solanacearum strains from heliconia and ginger was only 0.165, and strains from ginger showed little similarity with strains from all other hosts. The second analysis of 55 strains from ginger on different Hawaiian farms confirmed that they were distinct from race 1 strains from tomato. Strains from ginger also showed greater diversity among themselves in the second analysis, and the greatest diversity occurred among strains from a farm where ginger is frequently imported and maintained. Our results provide evidence that R. solanacearum strains from ginger in Hawaii are genetically distinct from local strains from tomato (race 1) and heliconia (race 2).

Genetic diversity of Carica papaya as revealed by AFLP markers

Genetic relationships among Carica papaya cultivars, breeding lines, unimproved germplasm, and related species were established using amplified fragment length polymorphism (AFLP) markers. Seventy-one papaya accessions and related species were analyzed with nine EcoRI-MseI primer combinations. A total of 186 informative AFLP markers was generated and analyzed. Cluster analysis suggested limited genetic variation in papaya, with an average genetic similarity among 63 papaya accessions of 0.880. Genetic diversity among cultivars derived from the same or similar gene pools was smaller, such as Hawaiian Solo hermaphrodite cultivars and Australian dioecious cultivars with genetic similarity at 0.921 and 0.912, respectively. The results indicated that self-pollinated hermaphrodite cultivars were as variable as open-pollinated dioecious cultivars. Genetic diversity between C. papaya and six other Carica species was also evaluated. Carica papaya shared the least genetic similarity with these species, with an average genetic similarity of 0.432; the average genetic similarity among the six other species was 0.729. The results from AFLP markers provided detailed estimates of the genetic variation within and among papaya cultivars, and supported the notion that C. papaya diverged from the rest of Carica species early in the evolution of this genus.

Virus Coat Protein Transgenic Papaya Provides Practical Control of Papaya ringspot virus in Hawaii

Since 1992, Papaya ringspot virus (PRSV) destroyed nearly all of the papaya hectarage in the Puna district of Hawaii, where 95% of Hawaii's papayas are grown. Two field trials to evaluate transgenic resistance (TR) were established in Puna in October 1995. One trial included the following: SunUp, a newly named homozygous transformant of Sunset; Rainbow, a hybrid of SunUp, the nontransgenic Kapoho cultivar widely grown in Puna, and 63-1, another segregating transgenic line of Sunset. The second trial was a 0.4-ha block of Rainbow, simulating a near-commercial planting. Both trials were installed within a matrix of Sunrise, a PRSV-susceptible sibling line of Sunset. The matrix served to contain and trace pollen flow from TR plants, and as a secondary inoculum source. Virus infection was first observed 3.5 months after planting. At a year, 100% of the non-TR control and 91% of the matrix plants were infected, while PRSV infection was not observed on any of the TR plants. Fruit production data of SunUp and Rainbow show that yields were at least three times higher than the industry average, while maintaining percent soluble solids above the minimum of 11% required for commercial fruit. These data suggest that transgenic SunUp and Rainbow, homozygous and hemizygous for the coat protein transgene, respectively, offer a good solution to the PRSV problem in Hawaii.

Differentiation, Distribution, and Elimination of Two Different Pineapple mealybug wilt-associated viruses Found in Pineapple

Surveys for Pineapple mealybug wilt-associated virus-1 (PMWaV-1) and PMWaV-2 were conducted on pineapple samples from Hawaii and around the world. Tissue blot immunoassays (TBIAs) with two different monoclonal antibodies (MAb) specific to either PMWaV-1 or PMWaV-2 indicated that both closteroviruses are widely distributed throughout the pineapple-growing areas of the world. In the worldwide survey, PMWaV-1 was found in 80% of the mea-lybug wilt of pineapple (MWP)-symptomatic and 78% of the asymptomatic pineapple plants tested. A subset of plants was tested for PMWaV-2; 100% of the symptomatic plants and 12% of the asymptomatic plants were positive for this virus. A reverse transcription-polymerase chain reaction (RT-PCR) assay was developed to differentiate between PMWaV-1 and PMWaV-2. Oligonucleotide primers were designed using distinct regions of the HSP 70 homolog genes of the two viruses. PMWaV-specific RT-PCR assays and TBIAs were used to screen the pineapple accessions maintained at the United States Department of Agriculture-Agricultural Research Service National Clonal Germplasm Repository for PMWaV infection; 73% of the accessions were found infected with at least one PMWaV. Pineapple accessions found PMWaV-free were challenged with viruliferous mealybugs to test for immunity to PMWaV-1. No immune germ plasm was identified. Potential alternative virus hosts were screened for infection with virus-specific RT-PCR assays and TBIAs and were also challenged with viruliferous mealybugs. No alternate hosts of PMWaV-1 or PMWaV-2 were identified. PMWaV-1 infection was eliminated through axillary and apical bud propagation from infected crowns. Strategies to manage MWP are discussed.

Use of a Tissue Blotting Immunoassay to Examine the Distribution of Pineapple Closterovirus in Hawaii

Specific monoclonal antibodies made to a pineapple closterovirus (PCV) were used in a tissue blotting immunoassay (TBIA) for the detection of PCV in pineapple. More than 2,000 samples were tested in 5 days by one person using this rapid and reliable assay. A survey was conducted using this assay to test more than 20,000 Hawaiian pineapple samples for the presence of PCV. PCV was detected in symptomless pineapple plants in the field and in the USDA pineapple germ plasm collection. Studies of the association of PCV with mealybug wilt of pineapple (MWP) suggest that PCV may be involved in MWP.