M. E. Stanghellini

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).

Efficacy of nonionic surfactants in the control of zoospore spread of Pythium aphanidermatum in a recirculating hydroponic system.

Zoosporic fungi are among the most destructive root pathogens in recirculating hydroponic cultural systems, and zoospores have been implicated as the primary, if not sole, infectious propagule responsible for the spread of these pathogens via the recirculating nutrient solution. In vivo experiments employing cucumbers as the susceptible host and Pythium aphanidermatum as the root pathogen demonstrated the efficacy of surfactants in the control of root disease caused by this fungus. Amending the recirculating nutrient solution with a nonionic surfactant (final concentration, 20 micrograms a.i./ml) resulted in complete control of the spread of the fungus via the recirculating nutrient solution. Zoospores were identified as the sole propagule responsible for pathogen dissemination in the recirculating nutrient solution. In the absence of a surfactant all plants within a recirculating hydroponic unit were killed within 5 to 6 weeks following hypocotyl inoculation of a single plant in the hydroponic unit.

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.

Control of root rot of peppers caused by Phytophthora capsici with a nonionic surfactant

Motile zoospores were identified as the sole infectious propagule of Phytophthora capsici responsible for spread of the pathogen in a recirculating rock wool cultural system. Amending the nutrient solution with a nonionic surfactant resulted in the elimination of zoospores and 100% control of the spread of the root pathogen from a point source. In the absence of the surfactant, all of the pepper plants within the cultural system, irrespective of plant age, died within 2 weeks following hypocotyl-inoculation of a single plant, which served as the source of secondary inoculum. The potential significance of surfactants for the control of polycyclic soilborne diseases attributed to Phytophthora spp. and other zoosporic pathogens is discussed.

Efficacy of biosurfactants in the management of Phytophthora capsici on pepper in recirculating hydroponic systems

Zoosporic pathogens are among the most destructive root pathogens in recirculating cultural systems, and zoospores are the primary infectious propagule responsible for their spread via the recirculating nutrient solution. More stringent regulation of pesticide usage in greenhouses has resulted in the need for alternative management strategies. This investigation was designed to evaluate the efficacy of rhamnolipid and saponin biosurfactants in managing zoosporic plant pathogens in recirculating systems where plants are cultivated hydroponically in either an inorganic or an organic substrate. The efficacy of biosurfactants in the control of root rot was demonstrated in vivo using a pepper – Phytophthora capsici pathosystem. Amending the recirculating nutrient solution with either a rhamnolipid or a saponin biosurfactant (150 and 200 µg a.i./mL, respectively), which selectively kill zoospores, resulted in 100% control of the spread of the pathogen. Disease control was achieved in both ebb and flow and top-irrigated cultural systems, with either an organic potting mix or rockwool as the planting medium. In the absence of either biosurfactant, all plants within a cultural system were killed within 6–7 weeks following hypocotyl inoculation of a single plant in the system, which served as the source of secondary inoculum. Injecting the rhamnolipid biosurfactant into the irrigation line during every irrigation also resulted in 100% control of the disease. These results provide evidence that biosurfactants may be suitable alternatives to synthetic surfactants and microbial agents for the management of diseases caused by zoosporic pathogens in recirculating systems.

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.

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.

Comparative protein studies of several Pythium species using isoelectric focusing

Buffer-soluble mycelial proteins of six species of Pythium were compared by isoelectric focusing. Species studied included P. aphanidermatum (P.a.), P. deliense (P.de.), P. myriotylum (P.m.), P. dissotocum (P.di.), P. violae (P.v.), P. ultimum var. ultimum (P.u.u.) and P.u. var. sporangiferum (P.u.s.). Each species had one or more distinct bands, in addition to many common bands of protein, when focused between pH 4-8. A higher concentration of proteins focused in the acidic region of broad pH range gels. Major protein bands were detected with Coomassie stain. Silver staining increased detection of proteins in low concentration (minor bands) in acidic, neutral, and alkaline regions. Other distinct bands of protein were identified for each species when protein samples were further resolved by focusing in the acidic region (pH 3-6). Several isolates each of P.a., P.u., and P.de., collected from various regions of North America, produced consistent protein banding patterns with minor variations. Although banding patterns of P. aphanidermatum differed slightly when isolates were cultured on different media, protein patterns were still specifically identifiable. No differences in protein bands were observed between P. u.u. and P. u.s. The utility of isoelectric focusing for the differentiation of species of Pythium is discussed.

Viability of oomycete propagules following ingestion and excretion by fungus gnats, shore flies, and snails.

Sporangia of Phytophthora capsici and P. nicotianae, as well as hyphal swellings of Pythium splendens, P. sylvaticum, and P. ultimum, were ingested by adult shore flies but none were viable after passing through the digestive tract. Oospores of Pythium aphanidermatum retained their viability following ingestion by adult shore flies. Larval stages of fungus gnats and shore flies ingested sporangia of Phytophthora capsici, P. nicotianae, and P. ramorum, but they were not viable upon excretion. In contrast, hyphal swellings of Pythium splendens, P. sylvaticum, and P. ultimum, chlamydospores of Phytophthora ramorum, and oospores of Pythium aphanidermatum, retained their viability after passage through the digestive tract of these larvae. Snails were capable of ingesting and excreting viable sporangia and chlamydospores of P. ramorum, which upon excretion infected detached leaves. Although the impact of larvae and snails in the rapid dissemination of pathogen propagules is unknown, this work does highlight the possibility that some often-ignored animal-fungus interactions should be considered in long-range dispersal of pathogen propagules via food webs.