Nematodes pose a serious problem for walnut and almond growers. Even with good pre-plant practices, such as proper sanitation and fumigation, these microscopic roundworms can still build up in the soil, attacking roots and impacting tree health.
But California growers who follow best practices for proper nematode management will see better production and healthier trees.
Nematode Effects on Tree Health
There are several types of nematodes that impact walnut and almond orchards in California:
- Lesion nematodes feed inside the root and are the most damaging nematodes to walnuts.
- Ring nematodes live in the soil and feed on the root tips.
- Root-knot nematodes cause swelling on the roots.
Nematodes can also transmit diseases like bacterial canker in almonds, a highly contagious disease that can threaten other healthy trees. Bacterial canker symptoms are most obvious in the spring and can include limb dieback on the tree, rough cankers and amber-colored gum that weaken the tree and can lead to total tree collapse. There may also be leaf spots and loss of young flowers, spurs and shoots.
Over time, the damage from nematode pressure can result in a gradual decline in production. Symptoms include general tree decline, lack of new growth and lower yield.
Testing for Nematodes
While visual inspection of roots can detect nematode damage, only an experienced lab can determine the presence, species and number of nematodes.
Samples should be collected by dividing the orchard into five-acre sections with similar history and soil type. Collect the soil samples around the tree where the soil is moist and roots are easily found. Soil and any live roots should be collected from six to 36 inches deep. Sample randomly across the block and place the samples into sealable plastic bags. Keep the bags cool and out of the heat.
Treat Nematodes to Protect Trees
Fumigation before planting reduces nematode numbers, but that is only a temporary fix. While there are not many post-planting treatments available, Movento® insecticide from Bayer is one option that can provide protection. Movento, applied as a foliar spray, has a two-way systemic action to manage nematodes from the leaves into the roots.
In addition to an in-season application of Movento in May or June during root flush, a post-harvest application of Movento provides residual control, treating existing populations and suppressing future infestations.
Research at the University of California, Riverside, department of nematology, has found that applications of Movento in established almond orchards can reduce nematode populations by 50 percent within five to six months after application. Additionally, almond trees treated with Movento in orchards that have nematode pressure have responded with more vigor and higher yields.
Research conducted at the Kearney Agricultural Center in Parlier, California, (including three walnut sites), showed a 50-percent reduction in lesion nematode populations over a five to six month period with various applications and timings. Movento was effective in orchards growing on Northern California black and Paradox rootstocks.