DATE OF PLANTING:

For each nematode species, there is a minimum temperature at which the nematodes are able to penetrate plant roots.

This table summarizes information available in the literature for important California nematodes.

If planting is to take place at a time of year when temperatures are below this infection minimum, or when soils are cooler, less damage can be expected than when soil temperatures are optimum.

Roberts and others have provided good demonstrations in the literature of how utilizing temperature at the time of planting can reduce nematode damage on carrots and wheat. This graph of soil temperatures over the period of a year relative to the minimum temperature at which M. incognita is able to penetrate plant roots demonstrates how altering the date of planting by only a few weeks can reduce nematode damage.

One main drawback to utilizing this technique can be in situations in which date of planting is driven by market values. It is generally more profitable to sell a particular crop when it is in short supply than when it is plentiful. Growers will try to time harvests to coincide with times they think it will be most profitable to market the crop. This timing may make it necessary to plant at times when nematodes are active.

TIME OF HARVEST:

NEMATODE DEGREE DAYS PER GENERATION:	DD	BASE
ROOT KNOT (MELOIDOGYNE SP.)
M. INCOGNITA	600	10C
M. CHITWOODI	600	5-6C
SUGARBEET CYST
HETERODERA SCHACHTII	450	8C

In some situations, altering the time of harvest can significantly affect the amount of nematode damage experienced. An early harvest can also prevent additional nematode reproduction and buildup as illustrated by the following example from the Phytonematology Study Guide: Sugarbeet cyst nematode can produce up to five generations on sugarbeet in the Imperial Valley in one growing season. Three of these generations occur between spring and early summer; therefore, if an infested field can be scheduled for an early harvest in March or early April, the third, fourth, and fifth generations can be avoided. Limiting nematode buildup in this way is important for rotation plans and for effective control of nematodes by chemical treatment before the next susceptible crop.

For early harvest technique to be utilized successfully, there needs to be some ability to predict when nematode populations will reach levels at which the economics in a particular field will tip in favor of an early harvest.

The discipline of entomology has perhaps made the greatest use of the degree day or heat unit concept for predicting population cycling of pest populations. An excellent summary of this concept can be found on the UC IPM server.

For plant parasitic nematodes, this concept is less developed but can still provide useful information for nematode management. Within California, enough information is available on two species of root-knot nematode and for sugarbeet cyst nematode to utilize nematode degree days.

This graph illustrates how the different life cycle stages of root-knot nematode correlate with increasing numbers of degree days.

For California, we can look at how altering the date of harvesting potatoes can affect damage due to Columbia root-knot nematode. Note in the chart above that although both M. incognita and M. chitwoodi have 600 degree days per generation, there are differences in the base temperature or the temperature at which reproduction can occur. For the purposes of our discussion, we can utilize a simple equation to demonstrate how degree days can be determined.

DEGREE DAYS ( OR HEAT UNITS) = (DAILY HIGH + DAILY LOW )/2 - DEVELOPMENTAL THRESHOLD [OR BASE]

For each day, one would add the high and low temperature, divide it by two and subtract the base temperature to obtain degree days for that day.

The UC IPM server has programs which will automatically calculate degree days based on information obtained from weather stations.

Let's take a look at the weekly progression of a population of Columbia root-knot nematode on potatoes in the Tulelake/Klamath Basin of Northern California. The presence of adult female nematodes under the skin results in a rough surface blemishing (A indicates healthy tubers, B indicates infested tubers).

Populations of infective juveniles (J2) at planting in May are typically very low and in some cases nondetectable even though a field may have a history of infestation. Note that the number of nematodes (second stage juveniles) per liter of soil (vertical) axis is a logarithmic scale so numbers are larger than they might first appear. The horizontal axis is in terms of nematode degree days determined from soil temperatures.

Although it has been determined that a single generation has 600 degree days, the first major population jump indicating a new hatch of infective juveniles appears at about 1,000 degree days. In this graph, degree days have been accumulated since planting even though it may take several weeks for substantial root development to occur to support nematode reproduction. The second major hatch of juveniles appears to occur at around 1,500 degree days.

Another possible interpretaton is that there actually is a generation at around 600 degree days, but the populations being measured are still low enough compared to the efficiency of the extraction technique that it is not evident.

Whatever the correct interpretation, it is evident that there is a tremendous increase in population between planting and harvest. If we next look at a graph of the effect of harvesting three varieties of potatoes on four different dates on the effect of nematode blemished tubers, we see that the earlier the tubers are harvested, the less blemish occurs. This correlates with additional data obtained during the date of planting trial indicating the greatest blemishing occurs following entrance of the later nematode generations into tubers.

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