Nematodes

Hosts

All fruit trees

Biology

Nematodes are worm-shaped, nearly microscopic organisms.  There are many species of beneficial nematodes that help to improve nutrient cycling and feed on other soil microbes including plant pathogens and insects.  There are a few plant parasitic nematodes (PPN) that affect fruit trees.  They feed on plant cells using a spear-like structure called stylet to withdraw plant juices.

Plant Parasitic Nematodes Important in Fruit Crops of the Intermountain West

Eight plant parasitic nematode genera have been found in fruit orchard soils in soil surveys in Colorado.  These are:  the root-lesion, dagger, spiral, root-knot, lance, citrus, ring, and stunt nematodes.  The most important of these are the root-lesion and the dagger nematodes.  There are currently no reports of plant parasitic nematodes affecting fruit trees in Montana.  

Root-Lesion Nematode (Pratylenchus spp.)

Root-lesion nematode can be a major cause of orchard replant failures. Lesion nematodes enter the root and burrow tunnels through the root cortex. Eggs are laid inside root tissues or in the soil.  They hatch, and the juveniles then enter the roots and contribute to root injury.  Root lesion nematodes are migratory and therefore are capable of repeatedly entering and exiting from root tissue. 

They cause small brown lesions on the white lateral roots and kill fine feeder roots.  The entire root system appears discolored when these lesions merge.  Severely affected trees may lose all feeder roots.  Ultimately, young replant trees will be stunted and chlorotic, and may die.  The most common root-lesion nematode in fruits that causes damage in apple, peach, cherry, grapes and so many other crops is P. vulnus.

Dagger Nematode (Xiphinema spp.)

This is the largest of the plant parasitic nematodes found in our orchard soils.  Up to 2-3 mm in length, it has a characteristic flanged bulb at the base of its stylet.  This nematode is observed in all most all fruit orchards in Colorado irrespective of crop, location, soil type, etc. 

Females lay eggs singly in the soil near plants. They may take 6-12 months to complete their life cycle from egg to adult.  Dagger nematodes do not burrow into roots, but instead insert their long stylet deep into root tips where they feed on root tip cells.  This feeding causes some necrosis and stunting and swelling of root tips.

Dagger nematode densities observed in most western Colorado apple orchards (18-152 nematodes/100 ml soil) are above the economic threshold level for apple (10-100 nematodes/100 ml soil).  Economic threshold levels are not known for other fruit crops.   

Eleven species of dagger nematodes are known to transmit 13 viruses, including Cherry Rasp Leaf Virus (CRLV) and tomato ringspot virus (TmRSV), which causes prunus stem pitting (PSP).  Some other important fruit viruses transmitted by dagger nematode are grapevine fan leaf virus, grapevine yellow vein virus, tobacco ring spot virus, peach rosette mosaic virus, and three strains of TmRSV causing peach yellow bud mosaic, prune brown line, and Prunus stem pitting.  The viruses are acquired within 24 hours of the initiation of feeding and are carried in the esophagus lumen for up to 12 months.  Virus particles are transmitted by both adults and juveniles.

Symptoms

Symptoms caused by most of the plant parasitic nematodes are difficult to distinguish from those caused by other soil related problems.  Thus, identification based on symptoms is not easy.  They include:  non-uniform plant growth, mostly in patches and with poor plant establishment, giving an unhealthy appearance; chlorosis and stunting (resembling symptoms of some virus infections); stubby roots, bifurcated roots, root lesions, root galls (knots); excessive root branching and proliferation (“hairy root” symptoms); and poor root health,  growth, and establishment.

Monitoring

Nematode populations and species within soils of production blocks are monitored by collecting soil samples in a representative pattern (usually as a transect or a zig-zag pattern) across the production block.  Approximately 1 cup (~250 ml) of soil is collected from the top foot of soil for each sample and placed into a plastic bag for transport to the extraction location.  A 100 ml subsample is taken from each sample and the nematodes contained are extracted by density centrifugation, elutriation, or wet sieving for enumeration and identification.   Once identified and counted, the results are compared to the economic damage threshold for that species, crop, and soil type.

Management

Most of the time, management of nematodes must focus on reducing nematode numbers to levels below the damage threshold.  However, management of nematodes is important as they are a predisposing factor to soilborne diseases and environmental stresses such as cold injury, frost, salt injury and/or micronutrient deficiency. 

Organic amendments will increase the beneficial nematodes which will help to reduce the PPNs.  Marigold, sudan grass and Brassica spp. can be used as green manure crops to control PPNs and boost free living nematode populations in the soil.  Glucosinilate or isocthiocyanate content in many brassicas is known to control many PPNs.

Soil solarization is very effective for control of many nematodes and other soilborne pathogens.  For soil solarization: plow field to ensure looseness, ensure adequate moisture, cover with plastic, seal the plastic to make it air tight and maintain the seal for at least 45 days in June and July.  Soil solarization combined with green manure crops should be more effective.

Host resistance

Many rootstocks are reported to have tolerance or resistance to PPNs.  In grape, rootstock 9407-14 is resistant to many PPNs.  Some Peach rootstocks are resistant to nematodes: Guardian is resistant to ring and root-knot nematodes, Nemagard is resistant to root-knot nematode, and Schwarzmann and Freedom rootstocks were rated as resistant against X. index. 

References

Pokharel, R. R.  and H. J. Larsen.  2007.  The importance and management of phytoparasitic nematodes in western Colorado fruit orchards.   Journal of Nematology 39(1): 96.

Pokharel, R. and H. J. Larsen.  2008.  Plant parasitic nematodes associated with fruit crops in western Colorado.  Western Colorado Research Center, Colorado State University.  Annual report, TR 07-08: 22-28.