DAMAGE THRESHOLDS

ROOT-KNOT NEMATODE ON PROCESSING TOMATOES - SAN JOAQUIN VALLEY:

NUMBER OF LARVAE/GRAM OF SOIL

A fundamental principle of pest management is that management or control is unnecessary unless the pest is at a level at which the pest is expected to damage the crop (damage threshold). In some pest management disciplines, such as entomology, the availability and use of damage thresholds and economic injury levels are common and at times leads to the assumption that this concept is equally valid for other disciplines. Although the concept is likely just as valid in nematology as in entomology, the ability to develop and utilize damage thresholds has proven to be fairly elusive. For many nematode-crop associations, damage thresholds have not been developed.

Root-knot nematode on processing tomatoes in the San Joaquin Valley of California is one example that can be used to illustrate some basic principles and problems. In later pages, we will also examine damage thresholds for sugarbeet cyst nematode

This table is adapated from the UCIPM Project Integrated Pest Management on Tomato manual.

If fields are sampled at harvest in late summer or fall, population levels in infested fields are usually easily detectable.

If fields are then left fallow or planted with a nonhost crop during the winter, the rate of population decline has been found to be approximately 85 percent.

Populations in the Spring are consequently much lower and in many cases may be below the detection level of normal methods of extraction.

A susceptible tomato crop is then planted, and populations increase until the fall, at the rates indicated. The table indicates the yields that can be expected based on the initial populations present in the field.

Note that these numbers are per gram of soil which is approximately one-tenth of a teaspoon.

If meaningful damage thresholds for nematodes on annual crops are to be developed, they will likely need to be based on samples taken at harvest for a following crop rather than shortly before planting which is the typical practice.

TEMPERATURE DEPENDENCE OF DAMAGE THRESHOLDS:

• CYST - (50F,10C)
  • COASTAL REGIONS (2-3 EGGS/GRAM IN COOL SOILS)
  • IMPERIAL VALLEY (1 EGG/GRAM)
• LESION - (59F, 15C)
• STEM & BULB - (59F, 15C)

Damage thresholds are often thought of as absolute numbers. For nematodes, at least, the temperature at the time of planting can be a major factor in determining rather or not damage will occur. If the temperature at the time of planting is below the activity or infectivity threshold (some examples are given in the table above) of the particular nematode of interest, less damage will be experienced than if temperatures are warmer.

The damage threshold for sugarbeet cyst nematode is higher in the coastal regions of California where soils are cooler than in the Imperial Valley of Southern California.

It is common practice to recommend that sugarbeets be planted when soils are cooler and less damage will be experienced from sugarbeet cyst nematode. This recommendation, however, is often in conflict with recommendations made by entomologists who recommend delaying planting until June to avoid aphid transmitted virus yellows disease.

P. thornei is known to cause serious problems on wheat in Mexico. It is commonly found in fields in Northern California but is not considered a problem on wheat. This is probably due to the fact that wheat is grown in the winter when soil temperatures are below the activity threshold of the nematode.

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