Abstract: A new method was developed to evaluate in vitro the systemic ability of fungicides against crown rot diseases caused by Phytophthora spp. Excised shoots of peach were inoculated with P. cactorum or P. citrophthora and inserted vertically into vermiculite, 2 cm deep, and flooded with test fungicide, in a storage jar. After incubation, shoots were removed from the vermiculite and stripped of their periderms and the length of necrosis was measured. The systemic fungicides metalaxyl and fosetyl-Al inhibited necrosis whereas copper hydroxide, captan and dimethomorph, which gave protection in bark strip and trunk inoculation assays had no effect. This confirmed that the new assay discriminated between systemic and non-systemic effects. The new method is not laborious, allows ample replications and is inexpensive.
Abstract: The effect of 10 calcium salts in controlling Phytophthora root rot of citrus was evaluated both in vitro, on agar and liquid media, and in vivo, on sour orange citrus seedlings. In vitro, calcium salts were applied at 300, 600, and 1200 ppm. Calcium oxide and calcium carbonate significantly reduced the growth of P. nicotianae, in comparison with the control; minimum dry weight of the mycelium was observed with calcium oxide; moreover, this salt significantly reduced zoospore production and their viability. In vivo, calcium salts were applied at 1200 ppm on 4-month old sour orange seedlings. Calcium propionate and calcium lactate significantly reduced the inoculum density of the pathogen in the soil, whereas all the other calcium salts—except nitrate, pantothenate and gluconate—significantly reduced root infections in comparison with the control. Phytotoxicity symptoms on sour orange seedlings were induced only by calcium chloride.
Abstract: American chestnut is one of hundreds of plant species plagued by root rot caused by Phytophthora cinnamomi. Phytophthora root rot is thought to have contributed to chestnut dieback prior to the arrival of chestnut blight, and it may now present a serious limitation to establishment of blight-resistant hybrid chestnut. We manipulated soil compaction and moisture to evaluate the effect of soil physical factors on incidence of Phytophthora root rot on American chestnut seedlings. Seedlings were grown under three watering regimes, two soil compaction levels and two fungicide levels. Increased soil moisture enhanced seedling growth in loose soil, but irrigation did not impact seedlings growing in compacted soils. Seedling mortality was greatest in wet, compacted soils. Disease incidence was highest in the wettest soils, irrespective of compaction level. Fine root necrosis and Phytophthora infection occurred on 58 and 24% of non-fungicide-treated seedlings, respectively. Presence of ectomycorrhizal fungi declined in compacted soils that were either wetter or drier than optimal. Occurrence of ectomycorrhizal fungal symbionts was unrelated to root rot. This study demonstrates the high susceptibility of American chestnut to this common root pathogen, even under moderate levels of soil compaction and moisture. While overcoming chestnut blight is the first step in restoring chestnut to its original range, to establish successful plantings it will be crucial to recognize and avoid sites where soil physical factors promote Phytophthora root rot.
Abstract: The total phenol levels in different olive organs and tissues are studied. The HPLC-MS studies point to the presence of oleuropein, catechin and tyrosol as some of the main phenolic compounds in these extracts. The effect of Brotomax treatment on phenolic compound levels in this plant and the possible role of these compounds as antifungal agents against Phytophthora sp. are also studied. An increase in the total phenol content of leaves and stems was observed 120 days after treatment with 0.3% Brotomax. The cortex was the stem tissue which showed the greatest accumulation of these secondary compounds. An in vitro study of the inhibitory effect of these compounds on fungal growth revealed that tyrosol was the most active agent, followed by catechin and oleuropein, their fungitoxic effect being greater when they acted synergically.
Abstract: The effect of temperature and bark injuries on the occurrence of crown rot of peach trees caused by P. cactorum and P. citrophthora were examined in field and laboratory. Lesions developed at 35 °C (the complete range of temperatures tested) but maximum development occurred at 20–25 °C. Greatest growth of these fungi on cornmeal agar (CMA) also occurred between 15 and 30 °C. Both pathogens could infect injured trees up to 20 days after wounding, but could not infect uninjured plants or plants wounded 40 and 30 days before inoculation, respectively. This study showed that temperature is a critical factor for the development of Phytophthora crown rot of peach trees. In addition, crown rot developed from recent wounds inoculated with agar plugs of Phytophthora.
Abstract: Field surveys undertaken in major tomato growing districts of the Karnataka state, located in southern part of India, revealed a high incidence of bacterial wilt caused by Ralstonia solanacearum and it is one of the most destructive bacterial diseases of economically important crops. Across all the tomato cultivars under evaluation, the disease incidence in plants ranged from 9% to 39% whereas the incidence in seeds ranged from 4% to 18%. The effects of tomato seed treatments with Pseudomonas fluorescens in the control of bacterial wilt under greenhouse conditions revealed that the treatments protected plants against soil-borne infections of the bacterial wilt organism. Seed treatment with antagonistic P. fluorescens strain significantly improved the quality of seed germination and seedling vigour. The disease incidence was significantly reduced in plants raised from P. fluorescens treated seeds followed by challenge inoculation with R. solanacearum. Periodic field surveys for the incidence of bacterial wilt of tomato could be recommended to monitor the populations of the bacterial wilt pathogen. Workable measures are presented that could lead to the reduction of the prevalence of this serious disease in affected fields of the small farm-holders.
Abstract: A simple method for application of phosphorous acid to control Phytophthora diseases was developed. The proposed method involves measuring equal weight of phosphorous acid and potassium hydroxide and dissolving phosphorous acid in water before adding potassium hydroxide. The pH of the neutralized phosphorous acid solution (NPA) is 6.2-6.7. The NPA concentration at 5000 mg/L was effective in controlling tomato seedling blight caused by Phytophthora capsici when using soil drench method. For suppression of lily leaf and blossom blight caused by Phytophthora parasitica foliar application is much more effective than soil drench. However NPA concentrations exceeding 2000 mg/L were phytotoxic to young plant tissues. The best results were achieved by applying NPA at the rate of 1000 mg/L for 3 times at one week intervals. Applying NPA to mother lily plants also effectevely protected bulblets from infection by P. parasitica during storage periods.
Abstract: Horticultural potting media have been amended with compost to enhance biological suppression and with Al2(SO4)3 to enhance abiotic suppression of plant pathogens, but these factors have not been simultaneously incorporated into the same medium. In this study, the efficacy of aluminum (Al)-amended potting medium containing 20% composted swine waste (CSW) was assessed for control of Phytophthora parasitica (syn. P. nicotianae2), a soilborne pathogen causing damping-off of many horticultural bedding plants. Steamed and unsteamed media were amended with no Al or Al at 0.0079 g of Al g-1 of medium with an Al2(SO4)3 solution at either pH 4 or pH 6. Infested leaf disks were buried for 2-day durations beginning 0, 6, 13, and 21 days after Al amendment. The number of sporangia produced on infested leaf disks was assessed. A similar experiment was conducted to determine the effect of steaming and Al amendments on pathogen populations. Medium treated with the pH 4 solution consistently reduced sporangia production between 38 and 65% on day 0, but no Al effect was noted at subsequent time points. The pH 6 amendment did not consistently affect sporangia production. Exchangeable Al levels decreased over time, and abiotic suppression was only observed at >2 µM Al g-1 of medium. Pathogen populations were occasionally affected by steaming and Al. Sporangia production in unsteamed medium was reduced by 50% on leaf disks buried on days 6, 13, and 21, but not on day 0. Al amendment of a 20% CSW potting medium enhanced suppression of P. parasitica and abiotic suppression occurred before biological suppression developed.
Abstract: Although phosphite has been effective in the control of P. cinnamomi in E. marginata (jarrah), the biochemical mechanisms behind phosphite protection are poorly understood. Using an aeroponics system, jarrah clones with moderate resistance to P. cinnamomi were treated with foliar applications of phosphite (0 and 5 g L−1). The roots were inoculated with zoospores of P. cinnamomi at 4 days before and 0, 2, 5, 8 and 14 days after phosphite treatment. Root segments were then analysed for activity of selected host defence enzymes (4-coumarate coenzyme A ligase [4-CL], cinnamyl alcohol dehydrogenase [CAD]) and the concentration of soluble phenolics and phosphite. Lesion development was most effectively reduced when phosphite concentrations within the roots were highest (i.e. days 8–14). During this time, the levels of host defence enzymes remained relatively unchanged. Lesion development was also effectively restricted when phosphite concentrations within the roots were lowest (i.e. days 2 and 5); a significant increase in host defence enzymes was associated with this decrease in lesion development. It was concluded from these studies that the effect of phosphite in controlling the pathogen is determined by the phosphite concentration at the host–pathogen interface. When phosphite concentrations within the roots are low, phosphite interacts with the pathogen at the site of ingress to stimulate host defence enzymes. At high phosphite concentrations, phosphite acts directly on the pathogen to inhibit its growth before it is able to establish an association with the host, and the host defences remain unchanged.
Abstract: There were two experiments in which three phosphite concentrations were sprayed onto the foliage of either Banksia brownii, B. baxteri or B. coccinea naturally infected with Phytophthora cinnamomi in the Esperance Plains bioregion of Western Australia, or B. grandis artificially infected with P. cinnamomi in Eucalyptus marginata forest of south-western Australia. Plots were either sprayed with one of three concentrations of phosphite (2.5, 5 or 10 g phosphite/L) and penetrant-spreading agent or sprayed only with penetrant-spreading agent. Mortality of the Banksia species, understorey and crown health, plant height and soil inoculum at each site were the dependent variables. There were no significant differences in overstorey phytotoxic necrosis rating of B. brownii, B. baxteri and B. coccinea between phosphite treatments 2.7 and 4 years after the first spray. The greatest phytotoxic effect of phosphite application was the stunting of B. coccinea growth, which did not occur in B. brownii, B. baxteri and B. grandis. For naturally infested Banksia species, greatest mortality occurred in B. brownii and least in B. coccinea, with B. baxteri being intermediate between mortality for B. brownii and B. coccinea. There was a significant reduction in apparent mortality rate and increase in years to 50% mortality following foliar application of phosphite to the four Banksia species. Greatest reduction in mortality of the Banksia species occurred for concentrations between 5–10 g phosphite/L. Challenge inoculation showed that spray treatments of 5 g and 10 g of phosphite/L were effective in B. grandis, 2.2 years after spray. The proportion of positive P. cinnamomi soil samples from the top 3 cm of soil was significantly greater than the proportion positive from 30 cm below the soil surface in infested B. brownii and B. coccinea, but there was no significant difference in the proportion of positive P. cinnamomi soil samples between depth of sampling for B. baxteri. Phosphite had no significant effect on the frequency of isolation of P. cinnamomi from the soil of infested B. baxteri and B. coccinea. Determination of phosphite effectiveness against P. cinnamomi and phytotoxic responses between plant species will assist prescription optimisation for the most effective protection of threatened flora.
Abstract