Organic Control of Root Rot in Blueberry
by Juanita Popenoe, Commercial Fruit Extension Agent
Root rot, caused by Phytophthora cinnamomi is a problem in blueberries in wet soils. Blueberry roots are shallow and like a lot of moisture, but when blueberry roots are too wet for too long, this disease organism can move in. Wilting and apparent nutrient deficiency symptoms on infected plants indicate the roots are not working correctly. Root rot is difficult to control with chemicals, and there is concern about resistance to the limited chemicals available, so researchers in Oregon looked for cultural ways to overcome this problem.
The researchers were working with northern highbush blueberries (a little different than our southern highbush blueberries, but just as susceptible to root rot) in a field infested with Phytophthora cinnamomi. They decided to compare changing the distance the irrigation drip lines were from the trunk since this can impact how wet the soil is around the base of the plant. Mulch type can also influence the moisture and temperature around the roots and base of the plant. Root rot likes warm, moist conditions, so they wanted to look at sawdust mulch vs weed mat geotextile mulch. Finally, calcium disrupts the infection process, but most calcium supplements will increase the pH of the soil too much. Gypsum will not appreciable increase the soil pH and provides more soluble calcium than carbonate sources such as lime.
The experiment was laid out to test all these options in combination. The double drip irrigation lines were placed either 8 inches out from the trunk, or right beside the trunk. The soil was either amended before planting with approximately 1 ton of gypsum per acre or nothing spread out in a foot-wide band on the beds. The beds were either mulched with about three inches of Douglas fir sawdust or black weed mat. A control treatment had no gypsum, was irrigated with narrow drip lines, and plants were treated with mefenoxam (sprayed on the surface of the beds shortly after planting in May and again the next year) and fosetyl-Al (sprayed on the leaves in late May each year). ‘Draper’ blueberry, a very susceptible cultivar, was planted as 3-L container stock 2.5 feet apart. An automated irrigation controller applied water based on plant size and evapotranspiration. Fertilizer was applied through the irrigation lines and the experiment was run for two years. Yields were not measured.
Weed mat increased root rot infection above levels found with sawdust mulch. The soil under the weed mat got hotter than under the sawdust, and plants required 20% more water in the first year with the weed mat, although plant growth was similar in both mulch types. Plants grown with a combination of wide drip lines and gypsum resulted in the best growth. Drip lines placed next to the crowns negated the disease-suppressive effects of gypsum by washing the calcium away from the root zone and providing a wetter root zone area for the fungus. However, wide drip lines, resulted in nitrogen deficiency in the first year because fertigation was applied too far out from the root zone of the liner plants. Conclusion: if nitrogen is properly managed, the use of gypsum and wide drip lines can help suppress phytophthora root rot in northern highbush blueberries and increase production in infested field soils. However, growth was still less than that with plants grown with fungicides.
The full article is: Nonchemical, Cultural Management Strategies to Suppress Phytophthora Root Rot in Northern Highbush Blueberry. HortScience 52(5):725-731. 2017. By: J.R. Yeo, J.E. Weiland, D.M. Sullivan, and D.R. Bryla.