First Report of Vine Decline of Mature Watermelon Plants Caused by Olpidium bornovanus.

In late May 2013, collapse of mature watermelon plants (Citrullus lanatus L.) at first harvest occurred in several drip-irrigated commercial fields in the Coachella Valley, California. Above-ground symptoms consisted of chlorosis, wilting, and death of leaves starting at the crown and progressing rapidly towards the tip of vines. Structural roots of collapsed plants appeared healthy but feeder roots exhibited a brownish discoloration. Microscopic examination revealed that almost all epidermal cells of feeder roots contained either sporangia or resting spores of a fungus tentatively identified, based upon morphological characteristics, as Olpidium bornovanus (Sahtiy.) Karling. No other fungi or fungal-like organisms were microscopically observed in or isolated from structural roots, feeder roots, or vascular tissue of collapsed plants. Leaf, root, and peduncle samples from collapsed plants were tested for Melon necrotic spot virus (MNSV), a virus known to be transmitted by O. bornovanus, and Squash vein yellowing virus (SqVYV), a whitefly-transmitted ipomovirus known to cause watermelon vine decline (1). No MNSV was detected using previously described methods (3). No SqVYV was detected by testing total RNA from symptomatic plants (RNeasy Plant Mini Kit, Qiagen, Valencia, CA) with reverse transcription-PCR using previously described primers and methods (1,2). Genomic DNA was extracted from zoospores of the fungus which were obtained from a single-sporangial isolate maintained on watermelon seedlings. Analysis of ITS 1 and 2 gene sequences and a subsequent search in NCBI GenBank revealed a 99% identity to nucleotide sequences for O. bornovanus (Accession Nos. AB205215 and AB665758). To confirm Koch's postulates, roots of three 5-day-old watermelon seedlings were inoculated by exposure to zoospores (~1 × 105) in a beaker for 2 min and then transplanted into pots containing vermiculite. Pots were irrigated daily and incubated in a growth chamber (25°C, 12-h photoperiod). Controls consisted of non-inoculated watermelon seedlings. The experiment was repeated twice. Within 15 days of inoculation, all inoculated plants were stunted, and roots of stunted plants were brown and most root epidermal cells were filled with either sporangia or resting spores of O. bornovanus. Within 30 days of inoculation, 40 to 60% of the inoculated plants died in all three experiments. No other microorganisms were microscopically observed in or isolated from necrotic roots. Control plants remained symptomless over the duration of the study. Although O. bornovanus has been reported as a root pathogen of melons in greenhouse conditions (3), this is the first worldwide report of the fungus as a root pathogen of watermelons and its association with a late season vine decline in the field. Near-saturated soil conditions resulting from a daily irrigation regime during the latter part of the growing season apparently favored extensive root colonization by this indigenous and opportunistic zoosporic fungus, suggesting that growers should exercise care regarding the duration and frequency of irrigation events. References: (1) S. Adkins et al. Phytopathology 97:145, 2007. (2) S. Adkins et al. Plant Dis. 1119, 2008. (3) M. E. Stanghellini et al. Plant Dis. 94:163, 2010.

Evaluation of Beauveria bassiana for management of citrus thrips (Thysanoptera: Thripidae) in California blueberries.

Citrus thrips, Scirtothrips citri (Moulton), is a plant-feeding pest most widely recognized for causing damage to citrus and mango fruits. This insect has broadened its host range to become a significant pest of commercial blueberries grown in the San Joaquin Valley of California. We evaluated Beauveria bassiana (Balsamo) for control of citrus thrips in blueberries grown under two watering regimes (drip irrigation with and without overhead sprinklers) and using two fungal formulations (commercially available spores in suspension vs. colonized seed) over two sampling periods, that is, for two 3-d periods after treatment. We found significant differences in thrips densities as a function of water regime treatment and fungal formulation. Thrips levels were reduced significantly with both fungal treatments at 3 d after treatment, but at 6 d, only results with colonized seed differed from the control treatment. These data suggest entomopathogenic fungi might be useful for control of citrus thrips on blueberries in particular situations (in organic production or as a resistance management tool) but that traditional pesticides will likely remain the preferred management option.

Pathogenicity and Management of Olpidium bornovanus, a Root Pathogen of Melons.

Greenhouse studies document, for the first time, that Olpidium bornovanus, an obligate, holocarpic, root-inhabiting zoosporic fungus heretofore regarded as a nonpathogenic parasite, is a root pathogen. Significant browning of the roots and reductions in shoot and root growth were recorded within 28 days following inoculation of melons with the fungus. Amending the recirculating nutrient solution with either a nonionic surfactant (Agral 90) or a strobilurin fungicide (azoxystrobin) resulted in efficacious management of the disease caused by the fungus.

Selective enhancement of the fluorescent pseudomonad population after amending the recirculating nutrient solution of hydroponically grown plants with a nitrogen stabilizer.

Fluorescent pseudomonads have been associated, via diverse mechanisms, with suppression of root disease caused by numerous fungal and fungal-like pathogens. However, inconsistent performance in disease abatement, after their employment, has been a problem. This has been attributed, in part, to the inability of the biocontrol bacterium to maintain a critical threshold population necessary for sustained biocontrol activity. Our results indicate that a nitrogen stabilizer (N-Serve, Dow Agrosciences) selectively and significantly enhanced, by two to three orders of magnitude, the resident population of fluorescent pseudomonads in the amended (i.e., 25 microg ml(-1) nitrapyrin, the active ingredient) and recycled nutrient solution used in the cultivation of hydroponically grown gerbera and pepper plants. Pseudomonas putida was confirmed as the predominant bacterium selectively enhanced. Terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rDNA suggested that N-Serve selectively increased P. putida and reduced bacterial diversity 72 h after application. In vitro tests revealed that the observed population increases of fluorescent pseudomonads were preceded by an early growth suppression of indigenous aerobic heterotrophic bacteria (AHB) population. Interestingly, the fluorescent pseudomonad population did not undergo this decrease, as shown in competition assays. Xylene and 1,2,4-trimethylbenzene (i.e., the inert ingredients in N-Serve) were responsible for a significant percentage of the fluorescent pseudomonad population increase. Furthermore, those increases were significantly higher when the active ingredient (i.e., nitrapyrin) and the inert ingredients were combined, which suggests a synergistic response. P. putida strains were screened for the ability to produce antifungal compounds and for the antifungal activity against Pythium aphanidermatum and Phytophthora capsici. The results of this study suggest the presence of diverse mechanisms with disease-suppressing potential. This study demonstrates the possibility of using a specific substrate to selectively enhance and maintain desired populations of a natural-occurring bacterium such as P. putida, a trait considered to have great potential in biocontrol applications for plant protection.

Disease Development on Lisianthus Following Aerial Transmission of Fusarium avenaceum by Adult Shore Flies, Fungus Gnats, and Moth Flies.

Fusarium crown and stem rot of lisianthus (Eustoma grandiflorum), caused by Fusarium avenaceum, is a destructive disease in California. The pathogen produces large masses of orangecolored macroconidia on stem lesions that extend up to 35 cm in length from the soil surface. Populations of macroconidia (97% viability) range from 1.1 × 108 to 1.9 × 108 per cm of infected stem tissue. An aboveground life stage for a soilborne pathogen could serve as a source for acquisition and aerial dissemination by adult shore flies, fungus gnats, and moth flies. Our results provide evidence that these three insects are attracted to and readily acquire (either externally and/or internally) macroconidia of F. avenaceum produced on naturally infected lisianthus stems and then disseminate acquired macroconidia to healthy plants, which subsequently died, or to an abiotic substrate (Komada's medium, KM). The high percentage of transmission, as evidenced by both the number of KM plates colonized by the pathogen (up to 68.5% within 18 h) and the number of plants infected (75% within 4 days), reflects the efficiency of these insects as vectors.

Reproductive Potential of Monosporascus cannonballus.

Vine decline of melons caused by Monosporascus cannonballus is a destructive disease worldwide. Ascospores, the only spore stage produced by this soilborne fungus, serve as the primary inoculum. Ascospore production on roots occurs primarily at the end of the cropping season, and high soil temperatures (25 to 30°C) govern, in part, the rate of reproduction of the pathogen. In vitro studies confirm that the optimal temperature for reproduction ranged from 25 to 30°C. Additionally, the root system of a single mature cantaloupe plant is capable of supporting the production of approximately 400,000 ascospores. The latter population, if incorporated uniformly into 0.03 m3 (1 ft3) of soil, would be equivalent to 10 ascospores per gram of soil. Known problem fields contain as few as 2 ascospores per gram of soil. These results offer a possible explanation for field observations that economically significant disease problems can occur after only two consecutive melon crops if environmental conditions are conducive to pathogen reproduction, and they suggest that strategies to inhibit reproduction would be instrumental in disease management.

Application of Preplant Fumigants via Drip Irrigation Systems for the Management of Root Rot of Melons Caused by Monosporascus cannonballus.

Root rot and vine decline, caused by Monosporascus cannonballus, is a destructive disease of melons in the desert production regions of southern California. In 1998, we initiated studies on the use of preplant fumigation to reduce resident pathogen populations in soil. Preplant fumigation with methyl iodide injected as a hot gas at 448.4 kg/ha through drip irrigation tape in preformed, tarped beds consistently provided significant (P < 0.05) reductions in the percentage of roots infected compared with the nonfumigated controls; these reductions were equal to or better than those achieved with an equivalent rate (448.4 kg/ha) of methyl bromide. Chloropicrin applied in water at 249.0 kg/ha through buried drip irrigation tape to either tarped or nontarped beds significantly (P < 0.05) reduced the percentages of both roots infected and roots on which perithecia were produced compared with nonfumigated controls.

Influence of Sub- Versus Top-irrigation and Surfactants in a Recirculating System on Disease Incidence Caused by Phytophthora spp. in Potted Pepper Plants.

Zoospores of Phytophthora capsici spread from inoculated source plants to healthy potted pepper plants located on separate ebb-and-flow benches when the recycled nutrient solution originated from a common reservoir. Amending the recirculating nutrient solution with a surfactant, which selectively kills zoospores, resulted in 100% control of the spread of the pathogen in an ebb-and-flow and a top-irrigated cultural system. Without a surfactant in the recirculating nutrient solution, all plants in an ebb-and-flow cultural system died within 6 weeks. In contrast, all plants in a top-irrigated cultural system died within 2 weeks after inoculation of source plants. These results suggest that the use of recycled irrigation water in an ebb-and-flow cultural system is less conducive to pathogen spread than its use in a top-irrigated cultural system, but may still serve as efficient means of inoculum movement in the absence of control measures.

First Report of Root Rot Caused by Pythium aphanidermatum on Artichoke.

In the deserts of Southern California, globe artichokes (Cynara scolymus L.) are grown as annuals. Greenhouse-grown seedlings (5 to 6 weeks old) are commonly transplanted into minimum-tilled, drip-irrigated beds in late summer (August to September) and harvested in winter and spring (December to April). By mid-October of 1997 and 1999, up to 30% of the plants in some commercial fields were either stunted or dead. There was no further progression of the disease over the remainder of the production season. Primary and secondary roots of symptomatic plants submitted for diagnosis in October of 1997 and 1999 exhibited extensive root rot. The organism consistently isolated from rotted roots that were plated onto water agar grew optimally at 37°C, produced inflated sporangia, intercalary antheridia, and oospores characteristic of Pythium aphanidermatum Edson (Fitzp.). Soil temperatures at the 10 cm depth during late summer range from 25 to 34°C. These high soil temperatures are known to be favorable to the pathogenic activity of P. aphanidermatum. To confirm Koch's postulates, 6-week-old artichoke seedlings were transplanted into potting soil that was artificially infested with the pathogen (20 oospores per g of soil). Oospores were obtained from 2-week-old V8 agar cultures of the fungus. Inoculated and noninoculated control plants were incubated at 28°C and the experiment was repeated once. All inoculated plants died within 2 weeks and P. aphanidermatum was recovered only from the rotted roots of inoculated plants. This is the first report of P. aphanidermatum causing root rot on artichoke.

Microbe-Mediated Germination of Ascospores of Monosporascus cannonballus.

ABSTRACT Ascospores of Monosporascus cannonballus germinated readily in the rhizosphere of cantaloupe plants growing in field soil. However, little or no germination occurred in the rhizosphere of melon plants growing in field soil that was autoclaved prior to infestation with ascospores. The latter data suggested that root exudates alone do not stimulate ascospore germination and that the soil microflora may be involved in the induction of ascospore germination. Amending field soil with streptomycin (which inhibits gram-negative microorganisms) did not suppress ascospore germination in the rhizosphere of cantaloupe plants. However, amending the soil with penicillin (which inhibits gram-positive microorganisms) did suppress ascospore germination. Pentachloronitrobenzene (PCNB), which inhibits the gram-positive actinomycetes but does not inhibit gram-positive or gram-negative bacteria, also suppressed ascospore germination. These results suggest that actinomycetes, either directly or indirectly, are involved in the induction of ascospore germination in field soil in the presence of exudates from cantaloupe roots. Optimum germination occurred at temperatures ranging from 25 to 35 degrees C, and data indicate that a high percentage (>/=72%) of the ascospore population within 500 mum of a root are capable of germination and subsequent penetration of cantaloupe roots.

Aerial Transmission of Thielaviopsis basicola, a Pathogen of Corn-Salad, by Adult Shore Flies.

ABSTRACT Chlamydospores of Thielaviopsis basicola were consistently observed in frass excreted by adults and larvae of shore flies that were collected in the immediate vicinity of naturally infected corn-salad plants obtained from a commercial greenhouse production facility. Approximately 95% of the adult flies and 85% of the larvae were internally infested with the pathogen. Pathogen-free adult shore flies were subsequently shown to acquire the pathogen by ingestion after feeding on naturally infected plants. Viable propagules of the pathogen were excreted by these internally infested adults and were capable of transmitting the pathogen to healthy seedlings, which subsequently became infected.

Effect of Phosphite on Tomato and Pepper Plants and on Susceptibility of Pepper to Phytophthora Root and Crown Rot in Hydroponic Culture.

Tomato and pepper plants were grown hydroponically in a greenhouse using phosphate or technical and commercial formulations of phosphite as sources of phosphorus nutrition to determine the effects on plant development and susceptibility to Phytophthora root and crown rot. Phosphite-treated tomato and pepper plants were deficient of phosphate and developed phosphorus-deficiency symptoms. Growth of plants (leaf area and leaf, stem, and root dry weights) that were fertilized with phosphite was significantly (P < 0.05) reduced compared with phosphate-fertilized plants. In Phytophthora capsici-inoculated pepper plants, incidence of Phytophthora crown rot was significantly reduced in phosphite-treated plants compared with no phosphorus or phosphate-treated plants. Incidence of crown rot in pepper plants treated with 1 mM phosphate plus 0.3 mM phosphite was intermediate between plants treated with only phosphite (1 mM or 0.1 mM) and plants treated with phosphate (1 mM).

First Report of Root Rot of Hydroponically Grown Lettuce Caused by Pythium myriotylum in a Commercial Production Facility.

Commercial cultivation of lettuce (Lactuca sativa L.) under hydroponic conditions was initiated in August 1997 in the U.S. Virgin Islands. One week following transplant into an outdoor growout system, several plants wilted and died. Over the next 2 weeks, approximately 50% of the 30,000 plants died and the remaining plants were severely stunted. Temperature of the nutrient solution ranged from 28 to 30°C. Root specimens received for diagnosis consistently yielded pure cultures of Pythium myriotylum Drechsler. To confirm Koch's postulates, lettuce seedlings were reared hydroponically in a greenhouse. Temperature of the nutrient solution ranged from 27 to 29°C. After 14 days, one plant in each hydroponic chamber (which contained six lettuce plants) was removed and the root system of the plant was placed in a beaker containing approximately 1,000 zoospores of P. myriotylum. After a 30-min incubation period, the artificially inoculated plant was replanted in the hydroponic chamber. Within 5 to 7 days all plants were severely wilted and exhibited extensive root rot. P. myriotylum was consistently reisolated from symptomatic plants. No wilt occurred on respective noninoculated plants. The above study was conducted three times with similar results. Although P. myriotylum has previously been isolated from hydroponically grown lettuce in experimental systems (1), this report demonstrates the destructiveness of this zoosporic pathogen in a commercial hydroponic lettuce production facility. Factors that contributed to the epidemic were an abundance of a susceptible host, a temperature regime optimum for pathogen growth and reproduction, and a mechanism for the rapid dispersal of the pathogen via the recirculating nutrient solution. Although the source of pathogen introduction into the facility is not known, the hydroponic system was located in an open field. Thus, the pathogen could have been introduced aerially via wind-blown dust, rain, or insects. We do know, however, that the transplants were pathogen-free. Reference: (1) A. C. Schuerger and K. G. Pategas. Phytopathology 74:796, 1984.

Incidence of Fusarium spp. in Asparagus Spears.

Fusarium proliferatum and F. oxysporum have been identified as causal agents of asparagus decline in the field and have been associated with reduction in spear quality (1,2). Our objective was to determine the origin and incidence of spear infection by these fungi during the cropping years 1994 to 1997. From 15 to 40 asparagus samples were randomly selected from the field, packing houses, and retail markets and assayed for Fusarium spp. Collections were made in California, Connecticut, Peru, Mexico, and Australia. The number of samples varied between sampling sites and for the time of harvest season. One Mexican collection site was sampled at the beginning, mid-point, and end of the harvest season to evaluate influence of decreasing carbohydrate levels and increasing temperatures on infection and growth of the fungi. Isolations included sections (5 to 6 cm) from the basal and terminal portions of the spear to evaluate postharvest growth in the spear. Fusarium spp. were recovered from spear samples that included all geographical sampling locations (mean 45%, range 20 to 90%). F. proliferatum was the dominant species isolated consistently from the desert areas regardless of harvest period. The frequency of F. oxysporum isolation ranged from 2 to 32% with no correlation to time or location of sampling. Basal sections were more frequently infected (94%) than terminal portions of the spear (less than 6%). No major differences in the percentage of infected spears were found in collection sites regardless of country sampled. There were differences in the incidence of infection between harvest sample dates. Spears sampled late in the harvest period were 57% more infected than spears of early or mid-season collections. Samples from the same fields, regardless of whether collected directly from the field, from packing sheds, or from the retail market, had higher infection rates later in the harvest season compared with earlier harvests. This may be attributable to warmer temperatures, changes in levels of carbohydrates, or other physiological factors. Spears harvested from the field that were infected with both Fusarium spp. had a greater incidence of infection than spears recovered from packing houses or from retail sources. This supports the theory that the source of spear infection is diseased crowns and not postharvest sources. Isolates from the spears of both Fusarium spp. were found to be pathogenic when challenged to asparagus seedlings. References: (1) R. G. Grogan and K. A. Kimble. Phytopathology 49:122, 1959. (2) W. Schreuder et al. Plant Dis. 79:177, 1995.

Identification and origin of plant pathogenic microorganisms in recirculating nutrient solutions.

Avoidance of root-infecting microorganisms was originally considered one of the advantages of cultivation of crops in a soilless, recirculating nutrient solution. However, to date, four viral, three bacterial and 21 fungal pathogens have been identified as causal agents of root disease in hydroponically-grown crops. Root-infecting fungi, particularly those which produce a motile stage known as a zoospore, have been the primary pathogens associated with extensive crop losses. Documented sources of these root pathogens in hydroponic systems include peat, surface water such as rivers and streams, and insects. The severity of disease caused by these introduced root pathogens is primarily governed by the genetic susceptibility of each crop and the temperature of the recirculating nutrient solution.

Remote detection of biological stresses in plants with infrared thermometry.

Green leaves of mature sugar beets infected with Pythium aphanidermatum and cotton infected with Phymatotrichum omnivorum had midday radiant leaf temperatures 3 degrees to 5 degrees warmer than adjacent plants with no sign of disease. The temperature difference persisted under varying conditions of soil moisture and could be used to detect biological stress imposed by these soilborne root-rotting fungi.