Australasian Association of Nematologists

Nematode monitoring can be useful as a management tool

Graham Stirling,Biological Crop Protection Pty. Ltd.

Nematicides and soil fumigants are widely used in horticulture, but de-registration of some of the most effective products and concerns about costs and off-target effects are causing growers to seriously consider whether a nematicide should be applied. Nematode monitoring provides objective data about nematodes and can be used to help growers make management decisions. This article outlines the monitoring system that is currently used in the Queensland pineapple industry.

Background information

For the last 30 years, almost every field planted to pineapples in Queensland was fumigated with EDB. However, it has now been withdrawn from the market and growers only have access to chemicals that are more expensive or less effective than EDB.

Despite the fact that EDB was universally used, previous research has shown that the importance of nematodes varies from field to field. In some fields, nematodes increase to damaging levels and control is only obtained by regularly applying nematicides, while in other fields, nematodes do not cause major problems. Monitoring provides a means of identifying fields that are most at risk from nematodes, as it does not make sense to use the same nematode control measures in all situations.

Monitoring involves regularly collecting a soil sample from a field and sending it to a laboratory for nematode analysis. Results will indicate whether nematode numbers are sufficient to justify nematicide treatment. If few nematodes are found, chemical costs can be reduced because nematicides are not applied unnecessarily. If higher numbers are present, it is often possible to minimise yield losses by applying a nematicide before severe damage occurs.

Nematode monitoring involves more than collecting an occasional sample from a field. Fields are sampled about once a year, and a bank of information is gradually built up for that field. With time, this historical data becomes more and more valuable, as it eventually enables results to be interpreted with more precision. Thus management decisions can be made with more confidence once fields have been monitored for several years.

• Protocol for collecting samples

1. Aim to collect four samples from a field during each crop cycle. The best times to sample are prior to planting, about 12 months after planting, at plant crop harvest and about ratoon crop harvest.
2. Select a field of 0.5-1 ha in size that is relatively uniform with regard to soil type, aspect, cropping history etc. Define the area with a block name or number, so that the same area can be identified in future.
3. Use a sampling tube about 2 cm in diameter. Walk over the area and collect 50 random cores of soil from the root zone (i.e. 0-25 cm depth). Carefully mix the soil in a bucket and retain a 500 ml sample for analysis. It will take about 20 minutes to collect a sample.
4. Ensure that the sample is kept cool prior to submission to the laboratory (i.e. keep samples in an insulated container and do not leave them in a vehicle parked in the sun). Do not refrigerate samples.
5. Pack samples in a Postpack box or other similar container and forward to:
6. Biological Crop Protection Pty Ltd
   3601 Moggill Road
   Moggill Qld 4070
7. Forward the following information with the samples:

Site or block name
Date previous crop ploughed out
Situation during intercycle (e.g. bare fallow, weeds or crop)
Anticipated planting date

Site or block name
Date planted
Nematicides applied before and after planting

Interpretation of nematode counts

Nematode management decisions should not be made on the basis of nematode counts alone. Data on root health and records obtained previously from a field (e.g. nematode monitoring data, notes on responses to nematicides, observations of symptoms caused by nematodes) should also be taken into consideration. The following is therefore only a general guide on how to interpret results of nematode analyses.

1. After a traditional bare fallow intercycle period of more than 3 months, nematode populations should have declined to low levels due to starvation. Root-knot nematodes should not therefore be detected and populations of lesion nematode should be very low. The presence of either nematode at planting suggests that either the fallow has not been long enough or that host plants (e.g. volunteer pineapples, weeds or a crop) have provided a food source and allowed nematodes to survive. If any nematodes are detected immediately prior to planting, a pre-plant nematicide should be considered and nematode populations should be checked again when the crop is 9-12 months old.
2. Nematodes often cause problems when virgin soil or fields that have previously been under pasture or another crop are planted with pineapple. A low or nil count in these circumstances should be interpreted with caution, as nematodes are sometimes present at depth and may migrate to the root zone after pineapples are planted.
3. A count of zero means that the nematode population is below the level of detection. However, this does not necessarily mean that nematodes should be ignored. For example, if samples were handled roughly or packed incorrectly, if they were exposed to excessive heat or the soil was very wet or very dry, or if previous experience suggests that nematodes should have been detected in a field, it is possible that nematodes may have died while being transported to the laboratory.

1. The following table is a guide to the likely hazard from root-knot nematode.

1. Nematode counts at about ratoon crop harvest can be used to indicate the likely hazard from nematodes in the next crop. However, samples at plant crop harvest are even more useful. Root-knot nematode populations of 0-20, 21-100 and >100 at any stage in the current crop suggest a low, moderate and high hazard to the next crop.

1. Root-knot nematode (Meloidogyne javanica) is the major nematode pest of pineapples in Queensland. Lesion nematode (Pratylenchus brachyurus) and reniform nematode (Rotylenchulus reniformis) can sometimes be important, but their economic thresholds are not well defined. If either of these nematodes are present at population densities greater than 100 nematodes/200 ml soil, they probably cause some crop loss.
2. Spiral, stubby root and ring nematodes are often present on pineapple, but they are not economically important.

Observations on root health

Root disease problems in pineapple are rarely caused by a single factor. Thus nematode monitoring is of limited value as a management tool without data on other components of the root disease complex. The best way to gain an overall picture of the root system is to monitor root health every time a field is sampled for nematodes.

Some information on root health can be obtained by making casual observations of roots, but there are advantages in assessing root health and recording the data in a formal manner. A monitoring system has therefore been developed in which five plants are selected at random from a field and each plant is rated for various parameters using a 1-5 scale. Recording books are available from Golden Circle Ltd. which contain data sheets on which ratings can be recorded. The rating system being used throughout the industry is as follows:

Although some growers will feel that a formal rating scheme is unnecessary, there are advantages in using such a scheme:

1. The process of observing roots provides an holistic view of the root system, while the ratings are useful as a management tool. The aim should be to achieve high ratings for root depth, root volume, fine feeder roots and new roots, and low ratings for galling, rotting and branching.
2. Root health data provides a useful record of changes in root health over the life of the crop.
3. During the process of completing a root health assessment, any major problems due to nematodes, Phytophthora, white grubs or symphylids will be diagnosed.
4. Root gall ratings allow growers to relate nematode counts from a laboratory to actual damage in the field. Such comparisons serve as a check on the results received from a laboratory, and enable the grower to gain confidence that the laboratory is providing meaningful results.

Using monitoring as a management tool

Once a root health rating has been completed and a nematode count is available, it should be possible to decide whether nematodes are affecting the current crop or are likely to cause problems in future. The best person to interpret such data is a pineapple agronomist who understands root diseases and is familiar with the field from which the data were obtained. Technical staff from Golden Circle Ltd. can provide such a service for company shareholders.

Quality control

It is important to note that all nematode densities mentioned in this paper refer to numbers of nematodes/200 mL soil after correction for extraction efficiency. If growers have samples processed by laboratories other than Biological Crop Protection, they should ensure that the laboratory has adequate quality control procedures and that their counts are corrected for extraction efficiency. Details regarding these issues can be found in the following publication:

Stirling, G. R.. Nicol, J. and Reay, F. (1999). Advisory services for nematode pests -Operational guidelines. RIRDC Publication No. 99/41, Rural Industries Research and Development Corporation, Canberra, 111 pp.

Acknowledgments

Some of the monitoring procedures outlined in this paper were developed while the author was employed by the Queensland Department of Primary Industries. Funding for that work was received from HRDC as project FR217/FR504. Current research is being supported by the pineapple industry through HRDC project FR96043.