Fighting Foliar Diseases Using Tools: Forecasters and Environmentally-Friendly Fungicides

M.K. Hausbeck, Michigan State University; M. Daughtrey, Cornell University, NY; L. Barnes, Texas A&M University

1. INDUSTRY NEEDS ADDRESSED:

   Disease epidemics of Botrytis, powdery mildew (PM) and downy mildew (DM) can occur on floriculture crops without warning and vary in severity. Fungicides currently in use are at risk due to development of resistance by the disease-causing organisms and pending regulatory action such as implementation of the Food Quality Protection Act. Further research on fungicides would not only identify effective products, but may provide new products with different modes of action that can be used in a management program aimed to delay the development of fungicide resistance in the pathogen. Increased knowledge regarding the influence of the environment on targeted pathogens and testing of disease forecasting systems currently available for agronomic crops may allow development of a system suitable for floriculture that would enable growers to time fungicide sprays and improve disease control.

2. RESEARCH CONDUCTED, SPECIFIC RESULTS, AND BENEFITS TO THE INDUSTRY:

   Epidemiology and management of PM : Atmospheric concentrations of Oidium sp. conidia in two research greenhouses containing infected poinsettias were monitored to investigate the role of environment in prompting conidial release and dissemination (see graph, below). Hourly concentrations of conidia of Oidium sp. were estimated using a Burkard volumetric spore sampler. The influence of temperature on disease development was studied by placing healthy poinsettias in each greenhouse for 7-day periods, removing them, and recording the days to the appearance of the first PM colony. Concentrations of airborne conidia of Oidium sp. were present in the greenhouses throughout the monitored period. We observed conidial release largely between 1000 and 1400 hours. A primary factor influencing the occurrence of conidial release events (CREs) was fluctuation of relative humidity (RH), often caused by watering. CREs occurred in response to both positive and negative changes in RH. Although many CREs occurred in conjunction with temperature changes, it was not required for conidial release. Temperature exceeded 25C for 21 days in May in greenhouse 2 (GH2) and the atmospheric conidial concentration was 20% of that occurring in April when only nine days exceeded 25C. Similarly, in greenhouse 11 (GH11), when the number of days the temperature exceeded 25C increased from 9 in February to 19 in March, the atmospheric conidial concentration was reduced by 75%. As a result, the PM epidemics in the greenhouses naturally lost intensity in March (GH11) and June (GH2). For the last five weeks of the study, temperatures in GH2 exceeded 30C at least two days per week. At the conclusion of the study in GH2, colonies were no longer developing on exposed plants and atmospheric conidial concentrations were declining such that fewer than 100 conidia/m3 were released in the final week of monitoring. In GH11, following the high temperature of 39C in March (week 7), atmospheric conidial concentrations were reduced (?833 conidia/m3/hour). Colonies did not develop on plants exposed during weeks 7 and 8, when temperatures exceeded 30C two to three days per week.

   The ability of PM to become epidemic seemingly at a time when poinsettias have colored bracts and are especially vulnerable to phytotoxicity and residues from fungicides has made disease management difficult. While thorough scouting for signs of infection is optimum for prompting fungicide sprays, many growers do not have the necessary personnel to accomplish the task. Some growers apply preventive fungicides during the entire production cycle until just prior to bract coloration. Results from this study may be helpful in the eventual development of a disease management strategy that utilizes environmental conditions to prompt fungicide sprays.

   A fungicide trial was conducted during 1999 at MSU testing various products for efficacy in controlling PM of poinsettia (see graph, right). The most effective treatments limited disease to fewer than three leaves/plant, and included the following: Latron B-1956 (2 fl oz), Systhane 40WP (4 oz), RH-0611 (2 lb), Cygnus 50WG (1.6 oz), and Cygnus 50WG (1.6 oz) + Latron B-1956 (1 fl oz). It was interesting to note that treating with RH-0611 completely prevented disease development. While adding Latron B-1956 to Cygnus 50WG reduced the number of infected leaves compared to Cygnus 50WG alone, the difference was not significant. The results of the trial indicated that there are several products both registered and under development that effectively manage PM.

   Epidemiology and management of DM: A Burkard spore trap and weather-monitoring equipment were placed two sites at one commercial grower-cooperator in Florida, and in a research greenhouse in California to monitor the development of a naturally initiated DM epidemic on snapdragon (see graph, next page). This is the second year that DM data has been collected from the commercial site in Florida. Rainfall, RH, temperature, leaf wetness, wind velocity and direction were measured in Florida, and temperature and RH in California. Average values of these parameters were recorded hourly. Site 1 in Florida was monitored from 23 November through 5 May, and site 2 from 4 January through 5 May. The California greenhouse was monitored from13 March through 16 April. Data analysis of these trials is ongoing. Preliminary results support the conclusions of 1999: spore release occurred mid-morning during a time when the relative humidity decreased and wind speed increased, and it followed a dew period which occurred the previous evening. The research areas included over ten different cultivars. Disease incidence was greatest on the 'Potomac Early Pink' and 'Rocket White.' Despite strong disease pressure on these varieties, infections were limited on 'Potomac Dark Orange,' 'Potomac Soft Yellow,' and 'Rocket Orchid.'

   

   A fungicide trial was conducted during 2000 at Cornell testing various products for efficacy in controlling DM of miniature rose, pansy and snapdragon. The most effective treatment on rose was Stature MZ, a new fungicide soon to be marketed by SePRO. This product contains a systemic (dimethomorph) and a contact fungicide (mancozeb) that is intended to slow development of resistance in the pathogen. Some statistically significant reduction in defoliation was seen with Compass (used alone) and Junction. All treatments reduced disease incidence significantly on snapdragons; Heritage 50WP (2 oz, 14-day) and Stature MZ completely prevented symptom development. DM on pansy was effectively decreased by all treatments, and Stature MZ and Junction prevented symptom development.

   Knowledge of the environmental factors that are favorable for disease development could alert growers that fungicide sprays need to be initiated or increased to avert or limit a DM epidemic. Growing cultivars which show some resistance to DM would help limit the disease, and using effective fungicides with different modes of action would delay development of resistance in the DM pathogen.

   Epidemiology and management of Botrytis:

   

   A fungicide trial was conducted during 1999 at MSU testing various products for efficacy in controlling Botrytis blight of poinsettia (see graph, next page). By the conclusion of the study, the untreated inoculated control had 18.2% of the leaves infected; nearly all of the infected leaves had sporulating Botrytis cinerea. Due to the variability among the replicates in the untreated inoculated control, no treatments were significantly different from this control in infected leaves (%) at the last observation date. However, disease developed on the untreated uninoculated control, likely due to the presence of phylloplane B. cinerea and environmental conditions favorable for disease development. Several treatments were significantly better than the untreated uninoculated control at the last observation date and included the following: Compass 50WP (1 oz), Medallion 50WP (1 oz), and A1584 11.25DF (2.22 lb), as well as the industry standard, Daconil 2787 4.17FL (2 pt). Daconil 2787 4.17FL successfully limited the infected leaves with sporulation (%) compared with the untreated inoculated control.
   A fungicide trial was conducted during 2000 at MSU testing various products for efficacy in controlling Botrytis blight of geranium (see graph, left). By the end of this study, there were no significant differences in percentages of diseased leaves. All fungicide treatments provided similar control of percentage of leaves with sporulating B. cinerea. Treatments that significantly decreased the number of leaves with sporulating B. cinerea and the total number of diseased and sporulating leaves compared to the water control included: Heritage 50WG (2 oz), and the industry standards, Daconil 2787FL (32 fl oz) and Chipco 26019 50WP (32 oz). This study identifies a reduced-risk fungicide, Heritage, that shows potential for controlling Botrytis blight of geranium.

3. PROFESSIONAL/PUBLISHED INFORMATION.

   Byrne, J.M., and M.K. Hausbeck. 1999. Management of powdery mildew (Oidium sp.) on poinsettia. Phytopathology 89:S99.

   Byrne, J.M., M.K. Hausbeck, and B.D. Shaw. 2000. Factors affecting concentrations of airborne conidia of Oidium sp. among poinsettias in a greenhouse. Plant Disease (accepted upon revision).

   Daughtrey, M., and M. Tobiasz. 2000. Evaluation of fungicides for control of powdery mildew of poinsettia. Fungicide and Nematicide Tests 55:(in press).

   Hausbeck, M.K., W.R. Quackenbush, and S.D. Linderman. 2000. Evaluation of fungicides for the control of powdery mildew of miniature roses, 1999. Fungicide and Nematicide Tests 55:(in press).

   Hausbeck, M.K., S.D. Linderman, and W.R. Quackenbush. 1999. Evaluation of fungicides in managing Botrytis blight of geranium, 1998. Fungicide and Nematicide Tests 54:536.

   Hausbeck, M.K. 1999. Strategies for finishing disease-free poinsettias. GM Pro 19(8):64-66.

This entry was posted on Monday, May 1st, 2000 at 12:34 pm and is filed under Disease Management, Progress Reports. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.