Epidemiology and Management of Walnut Blight

نویسندگان

  • J. E. Adaskaveg
  • H. Förster
  • D. Thompson
چکیده

Field studies focused on evaluating the efficacy of new bactericides for reducing the incidence of walnut blight, verification of XanthoCast as an effective model for forecasting the disease, and on evaluating host resistance among walnut genotypes under ambient and simulated-rain conditions. In studies with new bactericides in plots in Butte, Tehama, Solano, and Fresno Co., no material evaluated was as effective as copper-Manex under lowand high-rainfall conditions for management of copper-resistant populations of the walnut blight pathogen in California. The non-registered DOW bactericide formulation 918-48C was effective at low rainfall by significantly reducing the disease incidence from that of the control in the Solano and Fresno Co. plots, but was not effective in plots where high rainfall occurred. Improved agricultural formulations of the DOW bactericide have to be developed with increased persistence on plants before any additional evaluation. The bactericide peroxyacetic acid (e.g., Zerotol) showed promise at the low rate evaluated in each trial. Significant reductions in disease from the untreated control with efficacy similar to copper-Manex treatments occurred in Fresno, Solano, and Butte Co. In Tehema Co., the Zerotol low-rate treatment showed a statistical trend for reducing disease by 50%. High rates were less effective in each of the plots. Possibly high rates or high application volumes of this material were phytotoxic to plant tissue. In the last three years, we demonstrated that simulated rainfall studies were very effective in developing efficacy data. In the Solano Co. trials, single applications of an inoculum-reduction treatment (CopperManex-0.5% Breakthru) under ambient or simulated rain-conditions failed for a third year, demonstrating that multiple, in-season applications of bactericides are needed under favorable environments. Thus, with a high reproduction potential of the pathogen under high rainfall environments (simulated rain treatments), this strategy is not part of our management guidelines. XanthoCast, the internet-based walnut blight-forecasting program, was available commercially for a third year for growers. The 7-day and cumulative indices followed the actual disease progress in each of 3 locations. Correlations of observed and predicted XanthoCast values that were based on 3and 5-day weather forecasts provided by Fox Weather gave coefficients of determination or R values mostly between 0.72 and 0.88 for 12 locations. Thus, disease was accurately predicted for different specific microclimates recorded for each weather station for three years and the model was experimentally verified in simulated rain studies in Fresno, Solano, Butte, and Tehama Co. Using a 17-day temporal “yard-stick” to track environmental conditions back in time from actual disease progress values, three distinct infection periods were identified in Butte and Tehama Co. between mid-April and early May. Furthermore, the model and the forecast system allow for targeted bactericide applications based on walnut blight infection periods. Disease was high throughout the walnut growing regions of the state this year and XanthoCast called for two to four bactericide applications. In Butte Co., no significant differences were observed in disease incidence between a 9 half-row application program (0.35%) and the XanthoCast program where 3 applications of Kocide-Manex were made on every row (4%). Both programs significantly reduced the disease incidence compared to the untreated check at 17%. In Tehama Co., one mid-April application was missed before a critical infection period, but still the management program based on XanthoCast significantly reduced disease as compared to the untreated control. Simulated-rain studies were also very important in evaluating walnut genotypes at KAC. In these studies, cv. Payne was the most susceptible with 15% disease incidence, whereas Serr, Franquette, Cheinovo, and 90-28-30 had no detectable incidence of disease on fruit. Other varieties were statistically intermediate between Payne and the four varieties that had no diseased fruit.

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تاریخ انتشار 2004