Thomas A. Evans

RNA-Seq reveals infection-related global gene changes in Phytophthora phaseoli, the causal agent of lima bean downy mildew.

Lima bean is an important vegetable processing crop to the mid-Atlantic USA and is highly susceptible to the oomycete pathogen Phytophthora phaseoli, which causes downy mildew. Genetic resistance and fungicides are used to manage P. phaseoli and often fail. Currently, the molecular basis of the interaction between this host and pathogen is unknown. To begin to rectify this situation, we used Illumina RNA-Seq to perform a global transcriptome analysis comparing P. phaseoli growing in culture with P. phaseoli infecting its host. Sequence reads from a total of six libraries mapped to gene models from the closely related late blight pathogen, Phytophthora infestans, resulting in 10 427 P. phaseoli genes with homology to P. infestans and expression in at least one library. Of these, 318 P. phaseoli homologues matched known or putative virulence genes in P. infestans. Two well-studied classes, RxLRs and elicitins, were up-regulated in planta, whereas the reverse was true for another class, called crinklers. These results are discussed with respect to the differences and similarities in the pathogenicity mechanisms of P. phaseoli and P. infestans.

First Report of Mefenoxam-Resistant Isolates of Phytophthora capsici from Lima Bean Pods in the Mid-Atlantic Region

Phytophthora capsici Leonian, the causal agent of lima bean pod rot, was first identified as a pathogen of lima bean in 2002 (1) and poses a new threat to lima bean (Phaseolus lunatus L.) production in the Mid-Atlantic Region. The phenylamide fungicide mefenoxam (Ridomil Gold; Syngenta Crop Protection) is widely used in the region for controlling foliar and soilborne diseases caused by Oomycetes. Isolates of P. capsici were collected from lima bean pods from production fields in Delaware, Maryland, and New Jersey from 1998 to 2004. These isolates originated from survey samples of lima bean fields for another pathogen, P. phaseoli, in 1999 and 2000 and diagnostic samples were submitted to the Plant Disease Clinic. Isolates were from lima bean, except for one from pepper (basal stem). Identification was made on the basis of morphometric characteristics. No known sensitive or insensitive isolates were included in the evaluation. Single zoospore cultures were evaluated for mefenoxam sensitivity on V8 agar plates amended with 100 ppm of mefenoxam, a previously tested concentration (2). Seven-millimeter-diameter agar plugs of each isolate were cut from the edge of actively expanding cultures of P. capsici with a cork borer and transferred to three V8 agar plates amended with mefenoxam and three unamended V8 plates. The plates were arranged in a completely randomized design and incubated at 25°C in the dark for 3 days. After incubation, colony growth was measured in millimeters and averaged for the three replicate plates of each isolate and percent growth relative to the unamended control was calculated. Mefenoxam sensitivity was assigned according to methods of Lamour et al. (2). The experiment was repeated once, and also run with a treatment of 200 ppm of mefenoxam. Of sixteen isolates screened, nine were rated as sensitive, four were intermediately resistant, and three were resistant. There was no difference between the 100 and 200 ppm results, except for a slight increase in sensitivity for one isolate. A subsequent experiment tested five isolates at concentrations of 1, 10, 100, and 1,000 ppm. Results were consistent with previous tests, with resistant isolates exhibiting some growth at the highest concentration of mefenoxam. One resistant isolate was from a field in Delaware previously cropped to slicing cucumbers with a history of mefenoxam applications. The second was from Caroline County, MD, which is heavily cropped to pickling cucumbers and likely to have been exposed to mefanoxam applications for the control of fruit rot; the origin of the third insensitive isolate from lima bean is unknown. Mefanoxam usage on lima bean is usually limited to one foliar application of mefenoxam+copper hydroxide to control downy mildew in the fall crop in wet seasons. This study indicates that mefenoxam resistance is present in populations of P. capsici in lima bean fields in the Mid-Atlantic Region, presumably as a result of mefenoxam applications to other vegetable crops, principally cucurbits, which are planted in rotation with lima beans or from nearby cucurbit fields. Implementing strategies to minimize fungicide resistance in other vegetables is important to slow resistance development associated with this emerging pathogen on lima beans. Lima bean pod rot continues to be seen sporadically each year in fields with a history of P. capsici and abundant rainfall or excessive irrigation. References: (1) C. R. Davidson et al. Plant Dis. 86:1049, 2002. (2) K. H. Lamour et al. Phytopathology 90:396, 2000.

ECTOMYCORRHIZAL MANTLES AS INDICATORS OF HYDROLOGY FOR JURISDICTIONAL WETLAND DETERMINATIONS

Ectomycorrhizae are symbiotic relationships between soil fungi and higher plants. Evidence of the symbiosis is the presence of a ‘mantle,’ a hyphal layer that covers root tips, and a change in root morphology. The potential use of ectomycorrhizal mantles as hydrology indicators for wetland determinations was evaluated on the Delmarva Coastal Plain (Delaware and eastern shores of Maryland and Virginia, USA) over three seasons. In theory, the distribution of mantles with soil depth should vary from uplands to wetlands in most years, as mantle development is considered to be impeded by anaerobic conditions. At four forested locations, plots were set up in seasonally-saturated wetlands and adjacent uplands and drained wetlands (twelve sub-sites). Plots were evaluated according to the Corps of Engineers Wetlands Delineation Manual for soils, plant community, and hydrology to identify a jurisdictional classification. Hydrology was further addressed using automated monitoring wells (twice daily readings), and anaerobic conditions were confirmed via platinum electrodes. Plant roots (Pinus taeda was targeted) were sampled via spade slices in March and August each year and separated by depth: O horizon, 0–5 cm, 5–10 cm, 10–15 cm, and 15–20 cm. Roots were evaluated for the presence of mantles. A threshold depth of 5 cm was identified. From a total of 892 roots with mantles in uplands (including effectively-drained wetland sub-sites), 253 (28%) were found below the threshold depth. For wetlands (including one ineffectively-drained wetland), seven of 331 roots with mantles (2%) were found below the threshold depth. Temporal and spatial variability in mantle data was common; however, mantles consistently occurred at greater depths where seasonally high water tables were lower. We concluded that mantle depth has potential as a hydrology indicator.

Enhanced germination of primed mericarps of parsley (Petroselinum crispum Mill. Nyman ex A.W. Hill) limited by Alternaria alternata proliferation

Summary Osmotic priming (–1.5 or –1.0 MPa for 7, 14 or 21 d at 20°C) increased germination rates of ‘Italian Dark Green’ and ‘Moss Curled’ mericarps of parsley, but increasing the duration of priming caused a linear decrease in germination percentage. Fungal colonisation, predominantly by Alternaria alternata, was associated with decreased germination percentage and reduced hypocotyl and radicle lengths. The extent of pericarp damage and fungal colonisation during priming were rated visually from scanning electron micrographs. Pericarp degradation increased from “slight” with some cuticular damage after 7 d priming, to “moderate” with some mesocarp visible, after 21 d priming. Distribution of spores and hyphae of A. alternata increased from “covering less than 1% of the surface” on non-primed mericarps, to “light and uniform” after 7 or 21 d priming. A combination pre-priming (30 min) and post-priming (5 min) mericarp soak in 0.6% (v/v) sodium hypochlorite containing 0.015% (w/v) Tween-20 surfactant controlled fungal proliferation during 7 d priming, but not 21 d priming.

Yield of greenhouse-grown tomato in substrates containing coir and parboiled rice or burnt rice hulls

ABSTRACT Substrate components produced in the sub-tropics or tropics were combined in various proportions as substrates for the greenhouse production of ‘Big Beef’ tomato. Coir was mixed with parboiled rice hulls (PBH) or burnt rice hulls (BRH) at 0, 25, 50, 75, or 100% volume. Fruit fresh weights at equal proportions of BRH or PBH in coir were similar, with the greatest fruit yields achieved with 50 to 100% coir (0 to 50% PBH or BRH). Compared to 100% coir, PBH in coir decreased substrate container capacity and increased air porosity, while BRH in coir increased container capacity and decreased the air porosity. Pre-plant fertilisation of the PBH + coir substrates (superphosphate, gypsum, trace elements, calcium nitrate, and surfactant) increased fruit yield significantly. Substrates containing 50 to 100% coir with BRH or PBH yielded greater fruit fresh weights than was achieved in in 70% BRH + 30% gravel (a typical substrate used in the Dominican Republic), and reusing the 70% BRH + 30% gravel led to a 36% increase in fruit fresh weight. Fruit fresh weights in 100% coir or 75% coir plus 25% PBH or BRH were similar to those achieved in a commercial peat-lite. Herein we report on substrates made from tropical components that perform similarly to a peat-based substrate in the production of greenhouse-grown tomatoes.