Evaluating Drip Applications for Phytophthora Root Rot Management in Blueberry

Jonathan E. Oliver, Fruit Pathologist and Extension Specialist, Department of Plant Pathology, University of Georgia Tifton Campus

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Phytophthora Root Rot Causal Organism, Symptoms, and Disease Cycle

Phytophthora root rot, caused by the soilborne oomycete pathogen Phytophthora cinnamomi, is the most frequently diagnosed disease of blueberry in Georgia and is a common disease concern in blueberry production throughout the southeastern U.S. Though rabbiteye blueberries (Vaccinium virgatum) can be infected as well, cultivars of southern highbush blueberry (V. corymbosum interspecific hybrids) are particularly susceptible to root rot. This disease can dramatically reduce plant vigor (Figure 1A) as a result of the significant damage to the root system caused by the pathogen (Milholland and Oudemans 2017). Leaves of affected plants often turn yellow or red (Figure 1B), and eventually marginal leaf burn and defoliation may occur. Frequently, root damage from root rot results in the death of affected plants and can dramatically shorten the productive lifespan of southern highbush blueberry plantings.

Phytophthora Root Rot Management

Cultural practices are key for reducing root infection and managing Phytophthora root rot.  Ensuring adequate drainage through initial site selection and using raised beds are critical for preventing issues with Phytophthora root rot (Sial et al. 2024). In some areas of the southeastern U.S., such as eastern North Carolina (Bill Cline, NC State University, personal observation), these practices are reported to be sufficient to prevent root rot; however, in other areas, such as within the Coastal Plain region of southern Georgia, these cultural methods are simply not sufficient on their own to prevent losses from root rot. As such, commercial southern highbush blueberry growers frequently rely on the application of fungicides including mefenoxam and phosphonate products to manage Phytophthora root rot (Sial et al. 2024). Mefenoxam (the active ingredient in Ridomil Gold) and phosphonates including potassium phosphite (in ProPhyt), Aluminum tris (in Aliette), and mono and di-potassium salts of phosphorous acid (in K-Phite and Reliant) are typically applied either as a drench, through the drip, or in a foliar manner, depending on the specific label and the grower’s preferred application method. In addition to these materials, products containing another fungicide, oxathiapiprolin (in Orondis Gold 200 and Orondis Gold) have recently been labeled and recommended for root rot control on blueberry (Sial et al. 2024). Differences are known to exist between these products in terms of their activity. Previous trial work at the University of Georgia has shown that uptake by roots is critical for the efficacy of mefenoxam on blueberry, whereas foliar applications of ProPhyt are effective for root rot control, due to its systemic activity (Brannen et al. 2009). Likewise, additional trial work has also shown that foliar applications of another phosphonate fungicide, K-Phite, are similarly effective against Phytophthora (Oliver et al. 2021; Table 1).

Though the efficacy of foliar applications of phosphonate fungicides for root rot control has been demonstrated, little/no trial work has been carried out utilizing drip applications of phosphonate fungicides on blueberry. Nonetheless, many blueberry growers in the southeastern U.S. prefer to make applications through the drip. Accordingly, during the 2021 growing season, a field trial was conducted to assess available fungicides for Phytophthora root rot control and to determine the impact of application method on fungicide efficacy in blueberry.

Phytophthora Root Rot Field Trial Evaluation Methods and Results

To assess the efficacy of chemicals applied through drip applications, a research trial was carried out during 2021 at a commercial blueberry farm near Blackshear, Georgia (Oliver et al. 2022) on 3-year-old plants of southern highbush blueberry cultivar ‘Farthing’. The trial location had a history of infestation with P. cinnamomi, and disease development within the test plots during the trial was adequate for evaluation. Treatments consisted of: (1) Ridomil Gold SL (applied via drip), (2) ProPhyt (foliar), (3) ProPhyt (applied via drip), (4) K-Phite 7LP (applied via drip), (5) ProPhyt (foliar) and Ridomil Gold SL (applied via drip) and (6) an untreated control. A randomized complete block design was utilized with four replications or plots of each treatment. Each plot consisted of a single row of plants (~200 plants per row). Drip applications were applied on 12 March and/or 14 April, while ProPhyt was applied to foliage until runoff with a backpack sprayer (equivalent to 100 gal water/A) on 12 March (treatment 2 and 5), 14 April (treatment 2), and 30 April (treatment 5). Cultural practices were consistent with methods commonly observed in southern highbush blueberry production in the Southeast. Root samples were collected from each plot on 18 May and 30 Jun and evaluated for Phytophthora root rot incidence (% of infected root pieces). Fruit yield (total weight and average weight per berry) was evaluated on 28 April and 11 May. Plant size (height and canopy volume) was evaluated on 25 May.

A numerically higher root rot incidence (5%) was observed on the roots of the untreated control plants versus all other treatments, and significantly less disease was observed following treatment programs 3 (ProPhyt via Drip), 4 (K-Phite via Drip), and 5 (ProPhyt via Foliar and Ridomil Gold SL via Drip) relative to the untreated control (Table 2). No statistically significant differences were observed between treatments with respect to fruit yield (total weight or weight per berry) or plant size (plant height or canopy volume). No phytotoxicity was observed at any time following any of the treatments.

Conclusions and Recommended Fungicides for Phytophthora Root Rot Management

Previous trial work in Georgia blueberries had shown that foliar applications of the phosphonate fungicides ProPhyt (potassium phosphite) and K-Phite (mono and di-potassium salts of phosphorous acid) are effective against Phytophthora root rot (Brannen et al. 2009; Oliver et al. 2021). The results of the trial work described above further demonstrate the efficacy of drip applications of K-Phite for management of Phytophthora root rot on blueberry (Oliver et al. 2022). While many growers have already incorporated K-Phite and other phosphonates into their disease management program, the results of this trial validate the continued use of foliar and drip applications of these products for Phytophthora root rot control in Georgia.

For management of Phytophthora root rot in blueberry, several fungicides are likely to provide effective control (Table 3). Please note that some of these products (ProPhyt and Aliette) are only labelled for foliar applications and current labels do not allow for applications of these products through the drip on blueberry. Conversely, the labels of other products (Ridomil Gold, Orondis Gold, and Orondis Gold 200) allow for drip applications but not foliar applications on blueberry. Among the products currently recommended in the Southeast Regional Blueberry Integrated Management Guide (Sial et al. 2024), only K-Phite is labelled for both foliar and drip applications. Nonetheless, in some cases there are similar products which include the same active ingredients, so please consult the individual labels for specific instructions.

It is generally recommended that fungicides utilized for Phytophthora control be first applied around the time that plants begin to grow in the spring (when the spring root flush occurs). Fungicides that can be applied to foliage (ProPhyt, K-Phite, and Aliette), should be applied after leaf emergence (when “active” leaves are present) to allow for systemic uptake. Where there is high disease pressure (in wet conditions) or on young plants (which are more vulnerable to root rots), regular applications of fungicides throughout the season are likely to be necessary to protect plants from Phytophthora root rot. Unfortunately, chemical controls are NOT effective for reversing severe root rot damage on plants. Therefore, it is important to stay out in front of this disease with a proactive management program. Also, note that plant injury can occur if phosphonate fungicides are applied to heat or water stressed plants, in acidic water or with acidifying agents, or in tank mixes with copper and foliar fertilizers.

For additional recommendations for disease control in conventional blueberry production, please see the Southeast Regional Blueberry Integrated Management Guide which can be found at smallfruits.org. Fungicide availability, labels, and recommended rates change frequently and vary between states and localities. Please consult the various labels for rates, other recommendations, and precautions. Always refer to the labels of potential tank mix partners to determine if specific tank mixes are allowed or recommended prior to use. If you have further questions or concerns regarding the management of Phytophthora root rot on blueberry, please contact your local extension agent or specialist.

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