Alternatives to nematicides in fruit and vegetable crops
Lynton Vawdrey and Graham Stirling (formerly), Department of Primary Industries and Fisheries, Queensland
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Introduction
Nematodes are commonly found attacking a wide range of fruit and vegetable crops in most growing areas. For the past 30 years, most fruit and vegetable growers have been able to avoid serious losses from root-knot nematode (Meloidogyne spp.) by regularly applying nematicides. However, the future of these chemicals is uncertain as they rank among the most toxic materials used in agriculture today and are known to have adverse environmental effects. Research to find alternatives to chemicals used for control of root-knot nematodes was conducted at the Bundaberg Research Station.
Crop rotation and nematode populations
The initial aim of the project was to examine the impact of various crop rotation practices on the population dynamics of root-knot nematode. Samples were collected from 40 sites in the Bundaberg area and assessed for nematode infection.
Samples taken from the 10 most important crops in Bundaberg showed that zucchini and tomato were generally heavily infested with root-knot nematode, whereas nematodes were uncommon on sugarcane and beans. Eggplant, squash, capsicum, sweet corn, pumpkin and rockmelon were sometimes heavily infested but about two thirds of the samples from these crops had few nematodes.
These observations suggest that there are many vegetable fields in Bundaberg where nematode populations are insufficient to cause economic damage. Monitoring and crop rotations therefore have a place in nematode management.
Which vegetable crops tolerate root-knot nematodes?
The next phase of the research was to determine the multiplication rate of root-knot nematode on 12 vegetable crops in the Bundaberg area and assess the crops' relative tolerance to root-knot nematodes.
The first trial was planted in October 1993. Crops grown included tomato, rockmelon, watermelon, pumpkin, sweet corn and capsicum. Three treatments were applied:
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high rates of nematode inoculum
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low rates of nematode inoculum
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a nematicide
Good nematode reproduction during the trial indicated inoculum applications were successful.
Table 1 shows that while all the crops hosted nematodes, only capsicum and sweet corn were sufficiently tolerant of the infestation not to suffer a yield reduction. An 800-fold increase in nematode numbers occurred within the life of one tomato crop.
Table 1. Results of the spring host tolerance and nematode
multiplication trial
| Crop | Nematodes/200 mL of soil (pre-plant) |
Nematodes/200 mL of soil (at harvest) |
Root gall index | % Yield reduction |
| Tomato | 4 | 3,106 | 3.5 | 36 |
| Capsicum | 4 | 945 | 1.4 | 0 |
| Pumpkin | 4 | 1,731 | 3.6 | 20 |
| Rockmelon | 4 | 2,724 | 4.2 | 34 |
| Watermelon | 4 | 43 | 1.3 | 36 |
| Sweet corn | 4 | 853 | 0.3 | 0 |
Capsicum on the other hand, while hosting a smaller nematode population increase of only 250 fold, showed no yield reduction and a low root gall index. Sweet corn similarly showed it could host nematodes with a 200 fold increase in nematode population with no yield loss and very low root gall index. The watermelon result of a low nematode count and a low root gall index but a significant yield reduction was probably due to a heat wave induced plant failure rather than primarily a nematode infestation. The root gall index used a rating system of 0 (no galls) to 5 (severe multiple galling and deterioration of the roots).
The second trial was planted in April 1994. The same nematode and nematicide treatments were applied. Crops grown were tomato, capsicum, eggplant, Chinese cabbage, zucchini, squash, french bean, broccoli and sweet corn. Results (Table 2) again showed that all crops hosted nematodes but only capsicum, sweet corn and broccoli were tolerant enough not to suffer any yield reduction. The other crop types suffered only minor yield reductions even with high nematode populations. The 19 percent yield reduction in zucchini with fairly low nematode numbers was most likely due to severe mosaic virus infection in some plots.
Table 2. Results of the autumn host tolerance, nematode multiplication
trial
| Crop | Nematodes/200 mL of soil (pre-plant) |
Nematodes/200 mL of soil (at harvest) |
Root gall index | % Yield reduction |
| Tomato | 4 |
2,545 |
3.0 |
6 |
| Capsicum | 4 |
20 |
0.6 |
0 |
| Eggplant | 4 |
13,472 |
2.9 |
15 |
| Chinese cabbage | 4 |
179 |
0.7 |
5 |
| Zucchini | 4 |
193 |
1.4 |
19 |
| Squash | 4 |
494 |
1.0 |
3 |
| French bean | 4 |
38 |
1.2 |
7 |
| Broccoli | 4 |
130 |
0.6 |
0 |
| Sweet corn | 4 |
20 |
0.6 |
0 |
A combination of decreased root efficiency due to severe nematode infection and hot dry weather is the likely cause of the higher yield reductions in the spring trial. These results place in doubt the need to use chemicals to control root-knot nematodes during the cooler months of the year.
Organic matter and numbers of root-knot nematodes in tomato
Soil organic matter is known to be detrimental to nematodes. With the likely removal of currently used nematicides for economic and environmental reasons, the long term effects of organic matter on nematode populations was investigated as a possible alternative management option.
Laboratory and glasshouse experiments examined the inhibition effects of molasses on root-knot nematodes. Molasses was found to inhibit both egg hatching and juvenile nematode motility. This result indicates that the inhibition is probably due to antagonism towards nematodes by micro-organisms.
The suppressive effects of sawdust+urea, filter press, molasses, a green manure cover crop and a nematicide were compared in the field. The plots were maintained for three successive tomato crops to determine the long-term effects of such treatments. The effects of the organic soil amendments on root galling and nematode populations are shown in Table 3.
Table 3. Effect of amendments on host damage and root-knot nematode
populations
| Treatment | Root gall index | % Population increase/reduction |
| 1. Sawdust & urea |
0.2 |
-80.7 |
| 2. Filter press |
1.2 |
-5.7 |
| 3. Molasses |
1.5 |
+339.8 |
| 4. Treatments 1+ 2 + 3 at half rates |
0.8 |
+24.5 |
| 5. Green manure |
2.5 |
+5,238.3 |
| 6. Nematicide |
1.5 |
+996.3 |
| 7. Unamended |
3.2 |
+2,502.7 |
All treatments except the green manure crop appeared to give some control of root-knot nematode. The sawdust treatment noticeably reduced parasitic nematode incidence. However, if sawdust is to be used commercially as a root-knot nematode suppressant, careful attention will have to be given to preventing nitrogen deficiency problems. Molasses gave similar control to the nematicide. Filter press also suppressed nematode populations.
Although the results suggest a potential benefit from organic amendments in reducing nematode damage on tomatoes, more work will be required before these materials can be reliably used under commercial conditions to obtain nematode control.
Further information
The initial trial information was first published in the Queensland Fruit and Vegetable News, December 15, 1994. Later trial results were published in Australasian Plant Pathology, 25:240-246 (1996) and 26:179-187 (1997). Refer to these publications for more details on trial methodology and results.
DPI&F information and services
- To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Centre on 13 25 23 for the cost of a local call. The Customer Service Centre is open 8 am to 6 pm Monday to Friday (excluding public holidays); E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.
- Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.
Industry links
- Industry links is a page of links to various sites of interest to horticultural growers.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.