Skip to main content
ARS Home » Pacific West Area » Corvallis, Oregon » Horticultural Crops Disease and Pest Management Research Unit » Research » Publications at this Location » Publication #405362
Research Project: Disease Management in Small Fruit and Nursery Crops Based on Knowledge of Pathogen Diversity, Biology, and Environmental Effects

Location: Horticultural Crops Disease and Pest Management Research Unit

Title: Progress towards effective Phytophthora root rot control

Author
item Weiland, Gerald - Jerry
item Scagel, Carolyn

Submitted to: Digger
Publication Type: Trade Journal
Publication Acceptance Date: 9/13/2023
Publication Date: 11/1/2023
Citation: Weiland, J.E., Scagel, C.F. 2023. Progress towards effective Phytophthora root rot control. Digger. 67(11):41-44.

Interpretive Summary: Phytophthora root rot is a severe problem in many rhododendron nurseries and we've made a lot of progress in understanding this disease over the last ten years. We surveyed seven Pacific Northwest nurseries and found that P. plurivora and P. cinnamomi were primarily responsible for causing the disease. All P. cinnamomi isolates were sensitive to the two most popular fungicides, mefenoxam and phosphorus acid, and both fungicides were effective at reducing root rot damage. However, P. plurivora has developed fungicide resistance at several nurseries, explaining why disease control is sometimes poor. We also showed that fungicidal soil drenches are much more effective than foliar sprays at reducing root rot damage. Both mefenoxam and phosphorus acid must be placed near the roots to prevent infection. More recently, we investigated how nutrition and soil moisture affect root rot severity. Our findings show that increased nitrogen fertilization can exacerbate root rot, but that changing the amount of irrigation does little to influence disease. Therefore, growers should focus on trying to prevent disease rather than relying on cultural methods to reduce root rot damage.

Technical Abstract: Phytophthora root rot is a severe problem in many rhododendron nurseries and we've made a lot of progress in understanding this disease over the last ten years. We surveyed seven Pacific Northwest nurseries and found that P. plurivora and P. cinnamomi were primarily responsible for causing the disease. All P. cinnamomi isolates were sensitive to the two most popular fungicides, mefenoxam and phosphorus acid, and both fungicides were effective at reducing root rot damage. However, P. plurivora has developed fungicide resistance at several nurseries, explaining why disease control is sometimes poor. We also showed that fungicidal soil drenches are much more effective than foliar sprays at reducing root rot damage. Both mefenoxam and phosphorus acid must be placed near the roots to prevent infection. More recently, we investigated how nutrition and soil moisture affect root rot severity. Our findings show that increased nitrogen fertilization can exacerbate root rot, but that changing the amount of irrigation does little to influence disease. Therefore, growers should focus on trying to prevent disease rather than relying on cultural methods to reduce root rot damage.