Abstract: The effect of phosphite concentration on lesion development by Phytophthora cinnamomi in stems and roots of Banksia grandis and Eucalyptus marginata and in stems of Banksia coccinea was assessed during a 4.3 year period after stem injection of phosphite. Lesion length 6 weeks after inoculation was significantly less in roots of B. grandis trees that had been stem injected with three concentrations of phosphite (50, 100 and 200 g phosphite/l) at two rates (1 and 2 ml/cm of stem circumference) compared with the not-injected control. With the exception of B. grandis trees injected with 50 g phosphite/l, lesion length for the high rate was not significantly different to the low rate. In roots of E. marginata, lesion development in response to phosphite was different to that in roots of B. grandis; lesion length in roots did not differ significantly between phosphite concentration and rate. Lesion length and girdling in stems of B. grandis and E. marginata was significantly less in those injected with phosphite than in not injected stems. One year after injection, callus tissue had contained lesions in stems injected with phosphite. By 4.3 years after injection of both hosts there was a steep significant negative linear relationship between phosphite concentration and either lesion length or girdling, with greatest lesion development in not injected stems and least in stems injected with 100 g phosphite/l. Recovery of P. cinnamomi from lesion margins 1 year after injection, was significantly less in trees injected with phosphite than in not injected trees. The amount of plant death reflected containment of lesion extension and girdling, and reduction of recovery of P. cinnamomi with phosphite concentration; 4.3 year after injection there was a steep significant negative linear relationship between phosphite concentration and percentage of plant death. In contrast to B. grandis and E. marginata, there was a U-shaped non-linear relationship between phosphite concentration and effectiveness of phosphite in controlling lesion extension and girdling in B. coccinea. Containment of lesion extension and girdling with time was greatest for B. coccinea stems injected with 25 g phosphite/l, least for stems not injected, and intermediate in stems injected with 50 and 100 g phosphite/l. As in B. grandis and E. marginata, containment of lesion extension and girdling in B. coccinea with phosphite concentration was reflected in the amount of plant death. The non-linear response to phosphite of some plant species indicated that injected concentration for B. coccinea should not exceed 50 g phosphite/l, whereas injected concentrations of up to 100 g phosphite/l could be recommended for B. grandis. Longevity of action of phosphite for 4–5 years in native plant species after one injection makes phosphite injection a practical control option for the control of P. cinnamomi disease front extension and the protection of threatened flora. Research into the effect of factors affecting longevity of action of phosphite would facilitate optimization of timing of injection.
Abstract: Fungicides are applied by air, chemigation, and ground in the Columbia Basin of Oregon and Washington. These methods of fungicide application differ in deposition of fungicide to the canopy and cost. This study compared the alternate use of air and chemigation application of fungicides (AIRCHEM) with chemigation alone (CHEM), by either measuring chlorothalonil or manganese (mancozeb) amounts in three canopy levels (upper, middle, lower), both on leaflets and stems, after multiple fungicide applications on a 7-day schedule. Greater amounts of chlorothalonil or mancozeb were usually found on the leaflets in the upper and middle canopy locations from AIRCHEM compared to CHEM, the day of fungicide application and 7 days later. Deposition of fungicides on stems generally follow the same pattern as leaflets, but the amount deposited and maintained on stems was significantly less than leaflets. Mancozeb deposition in the three canopy levels followed the same pattern as was found for chlorothalonil. The greater the amounts of chlorothalonil on leaflets and stems resulted in better disease reduction during inoculation assays. Reduced fungicide amounts on stems compared to leaflets may be the reason for increased stems infections in recent years by more aggressive strains of late blight. This is the first report quantifying chlorothalonil or mancozeb amounts on potato stems and the first to report amounts of mancozeb on potato foliage after fungicide application.
Abstract: Four fungicides were evaluated for their effectiveness against Phytophthora diseases of peach trees and their ability to absorbed and translocated by roots of peach tree in laboratory. Meta-laxyl suppressed the development of P. cactorum and P. citwphthora on segments. Fosetyl-Al showed variable fungicidal activity against P. cactorum and P. citwphthora. Both dimetho-morph and cyraoxanil were not effective to inhibit the growth of fungi on segments. Section of tree trunks in 2-yr-old GF 677 trees were painted with one of the test fungicides. Strips of bark were removed 10 and 20 days after painting within treated area and inoculated with P. cactorum and P. citwphthora. Generally, the results agree with them obtained with excised stem and excised twig methods. Exception is the dimethomorph that reduced the development of P. cartorum and P. citrophthora. This study indicated that application with metalaxyl to the peach tree appears to be an effective procedure to control Phytophthora diseases on peach trees.
Abstract: Radial growth of Phytophthora cactorum and P. citrophthora was inhibited by T. harzianum and reduced by P. expansum and A. radiobacter, B. cereus did not influence the mycelia] growth of both Phytophthora species. Disease resistance on P. cactorum was induced by a local pre-inoculation with the non-pathogen P. megasperma, P. capsici, or P. drechleri. Besides, it was shown that crossprotection is not systemic but is localized in the area of original inoculation. The influence of heat and flood in the defense mechanisms of peach tree to P- cactorum, P. megasperma, P. capsici, and P. drechleri was also evaluated. Development of P. cactorum on plants was not influenced from any treatment. P. megasperma developed little necrosis only on stressed plants. In contrast, P. capsici and P. drechsleri had not the ability to infect heat and flood-treated plants. Metalaxyl and dimethomorph inhibited the mycelial growth of both Phytophthora species at rates of 10 ppm but not the growth of antagonists. I n contrast, fosetyl-Al inhibited radial growth of antagonists and both Phytophthera species at concentration as high as 500 ppm. Based on these results, metalaxyl and dimethomorph may be good candidates for integrated approach to manage Phytophthora diseases.
Abstract: Out of eleven fungicides, Ridomil, Aliette (systemic) and Euparen M were most effective in inactivating the fungus mycelium in the soil within 2 day. They also completely inhibited the production of sporangia and oospores. These fungicides and Antracol also reduced the viability (50–100%) of sporangia in the soil. They also reduced the viability of oospores along with Mancozeb, Zineb liquid Ziram. Euparen M, Antracol, Mancozeb, Zineb and liquid Ziram completely killed Zoospores; Ridomil, Previcur and Aliette were also effective. These fungicides can thus reduce the incoculum potential of the pathogen in the soil and reduce the chances of new infections.
Abstract: The effect of a soil amendment (SA) composed of urea (200 kg of N per ha) and CaO (5,000 kg/ha) on the survival of Ralstonia solanacearum in four Philippine soils was investigated in a series of laboratory experiments. Within 3 weeks after application, the SA either caused an initial decrease, a final decline, or no change in the pathogen population, depending on the particular soil type. An initial decrease occurred in a soil with a basic pH and resulted in a significantly (P < 0.001) lower pathogen population immediately and at 1 week after amending the soil. This decrease was probably due to the high pH in the soil during urea hydrolysis. A final decline in the R. solanacearum population after 3 weeks occurred in two soils in which nitrite accumulated after 1 week. In these soils, no decline in bacterial levels occurred when nitrite formation was inhibited by 2-chloro-6-trichloromethylpyridine. In the soil with low pH, no nitrite accumulated and the R. solanacearum population did not decline. The suppressive effects of pH and nitrite on R. solanacearum growth were confirmed by in vitro experiments. Ammonium reduced the growth of R. solanacearum, but was not suppressive. Interactions of pH with ammonium and nitrite also occurred, whereby ammonium reduced growth of R. solanacearum only at pH 9 and nitrite was suppressive only at pH 5. Nitrate had no effect on R. solanacearum growth in vitro.
Abstract: Heat shock inhibits pathogen-induced resistance mechanisms in incompatible plant hosts, leaving them vulnerable to pathogen attack. Prior exposure of organisms to non-lethal heat induces heat shock proteins and acquired thermotolerance to otherwise lethal high temperatures. The phenylpropanoid pathway is a target of heat-related inhibition but it is unknown whether thermotolerance protects this pathway or its key regulator, phenylalanine ammonia-lyase (EC 4.3.1.5, PAL). It was hypothesised that prior exposure to a heat shock pulse to induce the accumulation of heat shock proteins, specifically the 70-kDa heat shock protein (Hsp70 – inducible/Hsc70 – constitutive), would protect phenylpropanoid metabolism from heat-induced inhibition. The tomato, Lycopersicon esculentum L. cv. UC82B, transformed with PAL2-GUS, and Ralstonia solanacearum, biovar II, were used as incompatible host-pathogen model. A prior heat shock pulse induced significant accumulation of Hsp70/Hsc70 and enhanced cell viability. This protected the pathogen-activated phenylpropanoid pathway (PAL2-GUS activity, PAL enzyme activity, lignin deposition) from heat-induced inhibition and promoted cell survival after a subsequent prolonged heat shock. This study suggests phenylpropanoid metabolism as a target of Hsp70/Hsc70-related protection of the resistance response activated in tomato against avirulent strains of Ralstonia solanacearum from heat-induced inhibition during a concomitant heat shock.
Abstract: Mefenoxam is the systemic phenylamide fungicide, which is widely used in controlling phytophthora disease of tomato, cucumber, pepper and watermelon, etc. The dissipation behaviour of mefenoxam residues in watermelons and soils was studied. The whole watermelon, melon flesh and soil matrices of mefenoxam were analyzed by GC–NPD. At three different spiking levels mean recoveries and relative standard deviation from spiked samples in six replicated experiments for each matrix were in the range 89.6–98.2% and 1.5–8.1%, respectively. Under field conditions, mefenoxam dissipation rate was found to be faster in the whole watermelons than in the soils. The results showed that the half lives in whole watermelon and soil from Beijing were 3.9 and 10.0 days, respectively, and the half lives in whole watermelon and soil from Shanxi were 3.7 and 28.4 days.
Abstract: Application of organic amendments has been proposed as a strategy for the management of diseases caused by soilborne pathogens. However, inconsistent results seriously hinder their practical use. In this work we use an extensive data set of 2423 studies derived from 252 papers to explore this strategy. First, we assess the capability of a specific organic amendment to control different diseases; second, we investigate the influence of organic matter (OM) decomposition on disease suppressiveness; and third, we search for physical, chemical and biological parameters able to identify suppressive OM. OM was found to be consistently suppressive to different pathogens in only a few studies where a limited number of pathogens were tested. In the majority of studies a material suppressive to a pathogen was ineffective or even conducive to other pathogens, suggesting that OM suppressiveness is often pathogen-specific. OM decomposition in many studies (73%, n = 426) emerged as a crucial process affecting suppressiveness. During decomposition, disease suppression either increased, decreased, was unchanged or showed more complex responses, such as hump-shaped dynamics. Peat suppressiveness generally decreased during decomposition, while responses of composts and crop residues were more complex. However, due to the many interactions of contributing factors (OM quality, microbial community composition, pathosystem tested and decomposition time), it was difficult to identify specific predictors of disease suppression. Among the 81 parameters analysed, only some of the 643 correlations showed a consistent relationship with disease suppression. The response of pathogen populations to OM amendments was a reliable feature only for some organic matter types (e.g. crop residues and organic wastes with C-to-N ratio lower than not, vert, similar15) and for pathogens with a limited saprophytic ability (e.g., Thielaviopsis basicola and Verticillium dahliae). Instead, population responses of the pathogenic fungi Phytophthora spp., Rhizoctonia solani and Pythium spp. appeared unrelated to disease suppression. Overall, enzymatic and microbiological parameters, rather than chemical ones, were much more informative for predicting suppressiveness. The most useful features were FDA activity, substrate respiration, microbial biomass, total culturable bacteria, fluorescent pseudomonads and Trichoderma populations. We conclude that the integration of different parameters (e.g. FDA hydrolysis and chemical composition by 13C NMR) may be a promising approach for identification of suppressive amendments.
Abstract: There is growing interest in incorporating economic factors into epidemiological models in order to identify optimal strategies for disease control when resources are limited. In this paper we consider how to optimize the control of a pathogen that is capable of infecting multiple hosts with different rates of transmission within and between species. Our objective is to find control strategies that maximize the discounted number of healthy individuals. We consider two classes of host–pathogen system, comprising two host species and a common pathogen, one with asymmetrical and the other with symmetrical transmission rates, applicable to a wide range of SI (susceptible–infected) epidemics of plant and animal pathogens. We motivate the analyses with an example of sudden oak death in California coastal forests, caused by Phytophthora ramorum, in communities dominated by bay laurel (Umbellularia californica) and tanoak (Lithocarpus densiflorus). We show for the asymmetric case that it is optimal to give priority in treating disease to the more infectious species, and to treat the other species only when there are resources left over. For the symmetric case, we show that although a switching strategy is an optimum, in which preference is first given to the species with the lower level of susceptibles and then to the species with the higher level of susceptibles, a simpler strategy that favors treatment of infected hosts for the more susceptible species is a robust alternative for practical application when the optimal switching time is unknown. Finally, since transmission rates are notoriously difficult to estimate, we analyze the robustness of the strategies when the true state with respect to symmetry or otherwise is unknown but one or other is assumed.
Abstract