The 2009 Late Blight Pandemic in the Eastern United States - Causes and Results

The tomato late blight pandemic of 2009 made late blight into a household term in much of the eastern United States. Many home gardeners and many organic producers lost most if not all of their tomato crop, and their experiences were reported in the mainstream press. Some CSAs (Community Supported Agriculture) could not provide tomatoes to their members. In response, many questions emerged: How did it happen? What was unusual about this event compared to previous late blight epidemics? What is the current situation in 2012 and what can be done? It's easiest to answer these questions, and to understand the recent epidemics of late blight, if one knows a bit of the history of the disease and the biology of the causal agent, Phytophthora infestans.

First Report of Plectosporium Blight on Pumpkin and Squash Caused by Plectosporium tabacinum in New York

In early August 2004, pumpkin and zucchini squash (Cucurbita pepo) plants grown in conventional and organic commercial operations in Orange and Dutchess counties, respectively, showed spindle-shaped lesions on vegetative tissues and silver russeting and spots on fruit, typical of Plectosporium blight. Approximately 20% of pumpkin fruit were affected at this early time in yield development, while the zucchini planting had been abandoned due to disease. Symptomatic pieces of stem, petioles, and main leaf veins were excised, surface disinfected with 0.5% sodium hypochlorite, placed on one-quarter-strength potato dextrose agar, and incubated at 21°C with a 12-h photoperiod. Pale pink colonies with pinkish, hyaline, aerial mycelium developed from the tissues. When examined microscopically, simple and branched conidiophores with apical phialides were observed, as well as non- and one-septate ellipsoidal to slightly curved conidia that measured 7.5 to 13.0 × 2.5 to 3.3 μm. The fungus fits the description of Plectosporium tabacinum (van Beyma) M.E. Palm, W. Gams, & H.I. Nirenberg (synonyms Microdochium tabacinum (von Arx, 1984) and Fusarium tabacinum (Gams & Gerlagh, 1968) (1). Pathogenicity was tested on 10 seedlings each of pumpkin, zucchini, gourd (C. pepo), winter squash (C. moschata), and cucumber (Cucumis sativa). Plants were spray inoculated at the three true-leaf stage with a spore suspension at 10⁴ conidia/ml in water with 1% gelatin. Plants were held overnight in a moist chamber and then transplanted into 12-cm-diameter pots and kept in the greenhouse for the rest of the experiment. P. tabacinum was reisolated from all inoculated plants which completes Koch's postulates. Symptoms were noted 3 days after inoculation on pumpkin, zucchini, and gourd, with typical spindle-shaped lesions on the main stem, petioles, and main leaf veins (2). Symptoms developed after 1 week on winter squash, and lesions were mostly concentrated on the older portion of the stem with occasional lesions on the petiole and main leaf veins. Symptoms on cucumber, however, did not develop until 2 weeks after inoculation and appeared as an inconspicuous line of coalesced lesions on the ridges of the main stem only. These symptoms could easily be misidentified as physical abrasions from handling or from wind scarring. These results confirm the high susceptibility of C. pepo species, and indicate that other cucurbits are susceptible, albeit at a lower level. To our knowledge, this is the first report of P. tabacinum in New York. A voucher specimen has been deposited in the Cornell Plant Pathology Herbarium (Accession No. CUP 67504). References: (1) M. E. Palm et al. Mycologia 87:397, 1995; (2) T. A. Zitter. Microdochium blight. Page 28 in: Compendium of Cucurbit Diseases. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. The American Phytopathological Society, St. Paul, MN, 1996.

Tobacco mosaic virus in Jalapeno Pepper in New York

Jalapeno pepper plants (Capsicum annuum cv. Jaladuro) grown in Erie County, New York expressed chlorotic oak-leaf patterns along the primary vein of fully expanded leaves. Fruit had patterns of irregular ripening with a bumpy surface. Symptom expression was most obvious in August 2002, when leaf and fruit abscission occurred. Symptomatic fruit samples were tested by western blot analysis for the presence of Cucumber mosaic virus (CMV), Potato virus Y (PVY), Pepper mottle virus (PepMoV), Tobacco etch virus (TEV), and Tobacco mosaic virus (TMV). A positive reaction for TMV, but none of the other viruses, was observed. Symptomatic leaf samples were tested by Agdia, Inc. (Elkhart, IN) for Alfalfa mosaic virus, CMV, Impatiens necrotic spot virus, Pepper mild mottle virus, PepMoV, PVY, TEV, TMV, Tobacco ringspot virus, Tomato ringspot virus, and TSWV and for potyviruses using a group-specific test. The Agdia test confirmed that the pepper plants were infected with TMV. The pepper field where the original samples were collected was surveyed for TMV-infected plants. Fifty symptomatic plants expressing foliar and fruit symptoms similar to those originally tested, and 50 asymptomatic plants were sampled by collection of three leaves per plant and tested using enzyme-linked immunosorbent assay for the presence of TMV. All symptomatic plants and 18% of asymptomatic plants tested positive for TMV. To our knowledge, this is the first occurrence of TMV causing losses in commercially grown pepper in New York.

Monogenic Dominant Resistance to Gummy Stem Blight in Two Melon (Cucumis melo) Accessions

Two melon (Cucumis melo L.) accessions, plant introduction (PI) 157082 and PI 511890, reported to be resistant to gummy stem blight, a disease incited by the fungus Didymella bryoniae, were crossed with a susceptible parent to determine the inheritance of resistance. Resistance in both accessions is due to a single dominant gene, based on analysis of F₁, F₂, and backcross populations. Additionally, PI 157082 was crossed with PI 140471, the other source of resistance identified to date, to examine the genetic relationship of resistance found in these two sources. The frequency of susceptible individuals from the (PI 157082 × 140471) F₂ population was consistent with a 15:1 resistant:susceptible ratio, indicating that PIs 140471 and 157082 possess different resistance genes.

Characterization of Pathogenic Races of Fusarium oxysporum f. sp. melonis Causing Fusarium Wilt of Melon in New York

Forty-six isolates of Fusarium oxysporum f. sp. melonis obtained from soil samples throughout melon-producing areas in New York State were identified on the basis of pathogenicity and colony morphology. Physiological races 1 and 2 were identified by their reaction on a set of differential melon cultivars. Race 1 was widely distributed, occurring in six of the seven New York counties surveyed. Twenty-seven of the 28 race 1 isolates were associated with vegetative compatibility group (VCG) 0134, whereas one was incompatible with all known VCGs of F. oxysporum f. sp. melonis. Twelve out of 18 race 2 isolates were associated with VCG 0131, and occurred in four counties in eastern and western New York. Five isolates of race 2, associated with VCG 0130, were recovered from a farm in Washington County, as was a single race 2 isolate which was incompatible with all known VCGs of F. oxysporum f. sp. melonis. Restriction fragment length polymorphisms in the nuclear DNA revealed variability among the isolates examined, but race 1/VCG 0134 isolates from New York and Maryland were identical or nearly so, as were race 2/VCG 0131 isolates from the two states. These findings suggest a close relationship between the populations of F. oxysporum f. sp. melonis in New York and Maryland. Race 2 isolates were more virulent than race 1 isolates, based on the number of days to first symptoms and death of melon seedlings.

Helper virus-dependent replication, nucleotide sequence and genome organization of the satellite virus of maize white line mosaic virus

Virus like particles (17 nm in diameter) associated with maize white line mosaic virus (MWLMV) were shown to be a satellite virus of MWLMV (SV-MWLMV) on the basis of the following properties: (1) The SV-MWLMV was dependent upon the presence of MWLMV for replication in maize, while the latter virus could replicate independently of the SV particles. (2) No nucleotide sequence homology was detected between the SV-MWLMV and MWLMV, using complementary DNA probes prepared to the two RNAs, in a Northern blot hybridization analysis. (3) The RNA of the SV-MWLMV translated in vitro to produce a protein of the same Mr (24,000) as that found associated with the SV particles. This protein could be immunoprecipitated with an antiserum to the SV particles. And (4), there was no serological relationship between the coat proteins of MWLMV and the SV-MWLMV. The complete nucleotide sequence of the SV-MWLMV RNA (1168 nucleotides) was determined. The SV-MWLMV RNA contains a single open reading frame encoding a polypeptide of Mr 23,961. Computer analysis revealed no significant homology between SV-MWLMV RNA and any other viral or satellite RNAs. However, the putative SV-MWLMV capsid protein is predicted to share some structural features with the capsid protein of the satellite virus of panicum mosaic virus.