D. M. Ferrin

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.

In vitro insensitivity to metalaxyl of isolates of Phytophthora citricola and P. parasitica from ornamental hosts in southern California

In vitro sensitivity to metalaxyl was evaluated for 13 isolates of Phytophthora citricola and 26 isolates of P. parasitica recovered from ornamental hosts or soil from 12 locations in southern California. One isolate of P. citricola from azalea and two isolates of P. parasitica from periwinkle were insensitive to metalaxyl. Mean EC 50 values for inhibition of linear growth were 219.9 yg a.i./ml for the metalaxyl-insensitive isolate of P. citricola and 717.4 and 742.4 yg a.i./ml for the two metalaxyl-insensitive isolates of P. parasitica

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.

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.

In vivo expression of resistance to metalaxyl by a nursery isolate of Phytophthora parasitica from Catharanthus roseus

Populations of a metalaxyl-insensitive isolate of Phytophthora parasitica from a nursery in southern California were determined every 2 wk for 10 wk from the root zones of container-grown Cartharanthus roseus either treated or not treated with metalaxyl or fosetyl-Al. When compared with no treatment, metalaxyl applied every 4 wk as soil drenches at 18.8 or Jr.4 mg a.i./L and fosetyl-Al applied every 4 wk as foliar sprays at 4.8 g a.i./L had no effect on populations of the isolate (...)

Colonization of cantaloupe roots by Monosporascus cannonballus

Penetration of Monosporascus cannonballus into and growth within cantaloupe roots was studied using light and electron microcopy. Germ tubes penetrated the epidermis, and hyphae grew, without branching, almost directly to the xylem. The hyphae traversed the endodermis into protoxylem cells, and then grew extensively within the lumen of metaxylem vessels. Eventually, the hyphae grew back out into the cortical cells. A relatively low percentage of cells within both the cortex and xylem of lesions contained hyphae. The hyphae were generally localized within the lesion and could rarely be isolated more than 2 mm away from the margin of the lesion. Regardless of tissue type, hyphae were predominately intracellular. M. cannonballus appeared to be most similar to vascular wilt pathogens in its mode of parasitism, but does not spread via the vascular system to above-ground plant tissues.

Cyclic production of sporangia and zoospores by Phytophthora capsici on pepper roots in hydroponic culture

Production of sporangia and zoospores by Phytophthora capsici on roots of hydroponically grown peppers was shown to be cyclic in nature. Production of sporangia on peppers grown with a 12 h light : 12 h dark photoperiod was mainly diurnal: percentage of full sporangia began to increase between hours 8 and 10 of the dark cycle, the greatest number of full sporangia was present at hours 10–12 of the light cycle, and empty sporangia were most abundant at hours 4–8 of the dark cycle. Zoospore production was primarily nocturnal; populations began to increase at the start of the dark cycle and, in general, peaked sharply at hour 4. Altogether, 79.5% of the zoospores, sampled every 2 h over a 24 h period, were isolated from the hydroponic solution during the dark cycle. Extending the light cycle by 2 h delayed the peak in zoospore production by 2 h, so that it still occurred at hour 4 of the dark cycle. When peppers were grown under continuous light, no cyclic pattern of sporangia and zoospore production occurred. These findings could enhance the success of detection methods and practices for managing zoosporic pathogens in recycled irrigation water.

Root rot of Brachychiton populneus seedlings caused by Lasiodiplodia theobromae.

Lasiodiplodia theobromae was the causal agent in the death of several hundred 18-mo-old Australian bottle trees (Brachychiton populneus) at a nursery in southern California. Disease became apparent after the plants, which had been shipped bare root from Israel, were reported in California. The fungus infects the taproot, causing maceration and collapse of tissue and the eventual wilting and death of the plant (.)

Foliar blight and root rot of container-grown giant redwood caused by Phytophthora citrophthora.

Phytophthora citrophthora was isolated from the blighted foliage of container-grown giant redwoods (Sequoiadendron giganteum) grown under overhead irrigation at a nursery in southern California. The isolate caused both foliar blight and root rot when inoculated onto greenhouse-grown redwood seedlings. Scanning electron microscopy of the foliar blight stage showed that zoospores aggregate at and infect via stomata. Inoculation of the roots of 1-mo-old seedlings resulted in a significant reduction in the mean dry weight of the foliage after 3 mo (51.1 and 33.6 g for the uninoculated and inoculated treatments, respectively) and a change in the morphology and distribution of the roots within the pots (.)