Efficacy of management tools for control of Phytophthora plurivora leaf spot of Rhododendron, 2015

Authors: Press, Caroline; Rolfe, Kenneth - Kenny; Grunwald, Niklaus - Nik

Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 3/15/2019
Publication Date: 5/15/2019
Citation: Press, C.M., Rolfe, K.J., Grunwald, N.J. 2019. Efficacy of management tools for control of Phytophthora plurivora leaf spot of Rhododendron, 2015. Plant Disease Management Reports. 12:OT027.

Interpretive Summary: This study was conducted as part of the IR-4 Ornamental Horticulture program to evaluate fungicides and biopesticides for management of leaf spot of Rhododendron caused by Phytophthora plurivora. Treatments were applied to foliage except for SP2770, which was applied as a soil drench. Additionally, two SP2770 and one A21008A treatments were only applied once to the foliage on 21 Sep. Foliar treatments were applied to run-off using a hand-pump sprayer, while drench treatments received 17 oz of fungicide solution to the soil. Untreated control treatments received equivalents of water. During the trial, Rhododendron plants were grown in 1 gallon plastic pots maintained in a screen-house under 30% shade conditions with daily hand watering. Daily temperatures ranged between 64-78°F without supplemental lighting. Four single-plant replications were arranged in a randomized block design. Three-days following each fungicide application, three leaves were detached from each plant and inoculated with P. plurivora. One spot on the abaxial side of each leaf (to one side of the mid-vein) was wounded and inoculated with 20uL of a 3000 sporangia/ml suspension and left in containers to incubate. After treatment, containers were placed in a growth chamber with a 14 hour photoperiod at 68°F. Every day, leaves were sprayed with distilled water to maintain humidity and containers were rotated 180 degrees for light distribution. Total lesion area was assessed 10 days after each leaf inoculation. Subdue Maxx, Micora (after two applications), SP2770 (drench at 1.3lb rate with two applications) and A21008 (2.4 oz rate with one application) were the best treatments resulting in significant disease control as compared to the untreated inoculated controls. SP2770 was effective at the 2.6lb rate with one application on 14 Sep) and A21008 at the lower rates with two applications did not confer any significant protection to P. plurivora compared to untreated controls. No phytotoxicity was observed for any treatment.

Technical Abstract: This study was conducted as part of the IR-4 Ornamental Horticulture program to evaluate fungicides and biopesticides for management of leaf spot of Rhododendron caused by Phytophthora plurivora. Treatments were applied to foliage except for SP2770, which was applied as a soil drench. Additionally, two SP2770 and one A21008A treatments were only applied once to the foliage on 21 Sep. Foliar treatments were applied to run-off using a hand-pump sprayer, while drench treatments received 17 oz of fungicide solution to the soil. Untreated control treatments received equivalents of water. During the trial, Rhododendron plants were grown in 1 gallon plastic pots maintained in a screen-house under 30% shade conditions with daily hand watering. Daily temperatures ranged between 64-78°F without supplemental lighting. Four single-plant replications were arranged in a randomized block design. Three-days following each fungicide application, three leaves were detached from each plant and inoculated with P. plurivora. One spot on the abaxial side of each leaf (to one side of the mid-vein) was wounded and inoculated with 20uL of a 3000 sporangia/ml suspension and left in containers to incubate. After treatment, containers were placed in a growth chamber with a 14 hour photoperiod at 68°F. Every day, leaves were sprayed with distilled water to maintain humidity and containers were rotated 180 degrees for light distribution. Total lesion area was assessed 10 days after each leaf inoculation. Subdue Maxx, Micora (after two applications), SP2770 (drench at 1.3lb rate with two applications) and A21008 (2.4 oz rate with one application) were the best treatments resulting in significant disease control as compared to the untreated inoculated controls. SP2770 was effective at the 2.6lb rate with one application on 14 Sep) and A21008 at the lower rates with two applications did not confer any significant protection to P. plurivora compared to untreated controls. No phytotoxicity was observed for any treatment.