Scientific Research

Management of GSB on Watermelon Transplants with Seed Applied Fungicides and Field Fungicide Programs Involving Pristine
Dr. Kenneth Seebold1 and Dr. David Langston2, 1University of Kentucky at Lexington and 2University of Georgia at Tifton

Gummy stem blight (GSB), caused by Didymella bryoniae, is the most destructive disease of watermelons and cantaloupes in Georgia.  Losses to GSB are high each year, and the disease has proven difficult to control with fungicides and cultural practices.  Management options for control of GSB in the field are rotation, deep turning diseased tissue, avoiding irrigating that prolongs leaf wetness, and preventive fungicide applications.  Of these management options, preventive fungicide applications is the most effective.  In recent field experiments, Pristine (boscalid + pyraclostrobin) has proven itself to be the most effective fungicide for suppressing losses to GSB.  Our experiments focused on using the boscalid component of Pristine as a seed treatment fungicide for suppressing greenhouse disease spread and developing spray programs that maximized the efficacy of and other effective compounds while practicing good fungicide resistance management.

Seed treatment test.  Experiments were conducted in transplant production houses at Lewis Taylor Farms in Tifton, GA.  Untreated seed of watermelon cultivars ‘Mardi Gras’ (diploid) and ‘Tri-X 313’ (triploid), along with the cantaloupe cultivar ‘Athena’, were treated with boscalid (Endura 70WG)  at a rate of 25 g of active ingredient (a.i.) per 100 kg of seed, or tebuconazole (Folicur 3.6F) at 2.5 g a.i. per 100 kg of seed.  Pro-Ized red colorant (Gustafson LLC, Plano, TX) was added to the mixtures; a control seed lot of each cultivar was treated with distilled water and colorant only.  Thirty seed of each treatment (fungicide) were sown into 3 rows in one half of a 200-cell polypropylene plug tray (20 rows × 10 cells).  Two rows in the center of tray were not seeded and left as a buffer, and the process was repeated for the remaining half of the tray.  All seed in a given tray were of the same cultivar of watermelon or cantaloupe.  At the appearance of the first true leaf, GSB was injected into the crown of each of three adjacent seedlings located in the center of the three rows planted to each treatment.  This process was performed on seedlings in one half of the 200-cell tray; the seedlings in the remaining half were left uninoculated for the purposes of evaluating secondary spread of disease.  The severity of GSB was evaluated on inoculated and non-inoculated seedlings one week after inoculation using a 0-10 scale were 0=no symptoms, 1=trace symptoms, and 10=complete death of seedling. Boscalid significantly reduced the severity of GSB on inoculated ‘Athena’ cantaloupe and ‘Tri-X 313’ watermelon, while the tebuconazole seed treatment did not differ from the untreated control.  However, secondary spread (movement from inoculated to non-inoculated seedlings) was reduced ~89% on ‘Athena’, ~75% on ‘Mardi Gras’, and ~55% by boscalid (Fig. 1b).  Tebuconazole reduced secondary spread of GSB by ~27% on ‘Athena’ and ~70% on ‘Mardi Gras’ seedlings.  Boscalid applied as a seed treatment was more effective in preventing secondary spread of GSB on transplants than initial infections.  This was likely affected by the amount of fungicide present in seedlings, and by the high concentration of disease used to initiate the epidemic versus the lower amount that contributed to secondary spread.  If applied to seed at a higher concentration, tebuconazole might have been more effective at suppressing disease on inoculated seedlings; however, triazoles are known to have growth-regulator effects on plants.  This is why a low dosage of tebuconazole was chosen for the study.  Seed-applied fungicides such as boscalid or tebuconazole have the potential to be used as part of a program to manage GSB in the production of watermelon and cantaloupe transplants.  Successful management of GSB on seedlings would reduce significant losses to producers of transplants, and would also reduce a source of initial inoculum that contributes to epidemics of GSB in the field.  Because fungicides such as boscalid and tebuconazole target specific metabolic pathways in fungi, the risk of resistance development in populations GSB is high.  Further work is needed to define ideal rates of application to seed for these compounds, and to assess the potential for resistance to boscalid and tebuconazole in GSB.

Field tests. Field plots at Reidsville, Crisp Co., and Attapulgus, GA were chosen for testing fungicide programs. All plots were 30 ft long, were bordered on either side by non-treated border rows, and were replicated 5 times. Plots were sprayed weekly with a sprayer calibrated to deliver 40 GPA at 80 psi using TX-18 hollowcone nozzles. At Reidsville, Dry conditions during May and Jun followed by wetter-than-average conditions in late Jun and Jul resulted in late onset of disease in the trial.  In general, all fungicide programs were effective in reducing the severity of GSB compared to non-treated plots.  Alternating Bravo with Switch or Vanguard did not result in improved suppression of GSB compared to Bravo alone.  Bravo alternated with Pristine had significantly less disease at the end of the season than Bravo alone, Bravo alternated with Vanguard or Bravo alternated with Vanguard and Quadris.  Yields for all fungicide treatments were significantly higher than the untreated control.  No differences in yield were observed between any of the fungicide programs and Bravo alone.  At the Attapulgus location, wetter than normal conditions in June resulted in early disease onset and high severity of gummy stem blight in the trial.  All fungicide programs significantly reduced the severity of GSB, in terms of area under the disease progress curve (AUDPC), compared to non-treated plots and Bravo alone. Severity of GSB at the end of the season was also lower for all fungicide programs (except Bravo applied twice followed by an alternation of Bravo and Pristine) compared to the untreated control or Bravo only. The timing of the initial application of Pristine in a given program had no significant effect on overall severity of GSB, as determined by AUDPC; neither did the number of applications of Pristine.  Fruit weight per acre was higher in plots treated with Bravo alternated with Pristine; Bravo applied 2-3 times followed by an alternation of Pristine and Topsin tank-mixed with Dithane; or Bravo applied twice followed by an alternation of Pristine and Folicur as compared to Bravo alone or the untreated control.  Topsin tank-mixed with Dithane (or any mancozeb product) was effective for the management of GSB in watermelon and for enhancing yield when used as a late-season rotation partner for Pristine.  In Crisp Co., wetter than normal conditions and warm temperatures created favorable conditions for development of anthracnose in the trial.  Severity of anthracnose at the end of the season, as well as season-long severity AUDPC, was significantly reduced by all fungicide programs compared to the untreated control.  Anthracnose severity at the final evaluation of disease was significantly lower for Bravo applied 2-3 times followed by an alternation of Pristine and Topsin tank-mixed with Penncozeb compared to Bravo applied 3 times followed by an alternation of Pristine and Topsin, or Bravo applied twice and then alternated with Pristine and Folicur. None of the fungicide programs performed better against anthracnose than Bravo alone; however, these programs would likely be effective where other diseases, such as gummy stem blight or downy mildew, are present.

In general, results from 2004 indicate that Topsin tank-mixed with mancozeb is an excellent rotational combination to use with Pristine to manage foliar diseases of watermelons.  Folicur (although not labeled) is also an excellent material to rotate with Pristine, especially for gummy stem blight suppression.  All of these materials are safe alternatives to use late in the season and do not cause phytotoxicity to watermelon rinds.  Pristine at the 12.5 oz/acre rate appears to be marginally effective against anthracnose and higher rates are warranted as per label instructions.

Back