So far this season, Kathy Kelley Anderson, UCCE farm advisor for Stanislaus County, Calif., has seen no major problems in the walnut orchards.
“A lot of nuts have set. Cool weather in May produced good shoot growth. There’s not much pressure from codling moth, and I haven’t seen much blight, so that shouldn’t be a big issue.”
But, she has been fielding a number of calls from growers about phytophthora root and crown rot. Trees infected with fungal diseases appear drought-stricken early in the growing season. Leaves wilt and dry, but remain attached to the tree. Infected trees may die within the same growing season or decline for several years before succumbing to the disease. Typically, it kills young trees because of their smaller root systems and crown areas compared to mature trees.
“It’s not everywhere — only in certain orchards,” she says. “The disease usually shows up at this time of year, when the trees start growing and, because of it, trees start to collapse.”
It’s caused by excess water around the tree crown, either from winter rains or too frequent irrigation, which leads to waterlogged soils.
“You can’t manage the disease but you can manage water and infiltration to prevent prolonged soil saturation,” Anderson says.
Preventive steps include:
- Planting walnuts on soil with good surface and internal drainage.
- Not over-irrigating in spring and fall when soil temperatures are most conducive to development of the disease and tree water use is low.
- Avoiding planting walnuts in soil invested with phytophthora species.
- Using Paradox rootstock in soil infested with phytophthora.
- Being careful not to introduce the pathogen into uninfected soil though nursery material, irrigation water or equipment.
- Avoiding high-angle sprinklers that hit trunks and limbs, especially if the water is from a surface source.