Tomatoes are usually one of the most rewarding vegetable to grow in your backyard, however their fussy tendencies and willingness to up and die at their peak makes me wonder why Ibother. This year I thought I had it right; I rotate the crops accordingly & planted a cover crop of mustard and legumes, added compost. Then at planting I added calcium aged manure and aspirin deep in the planting hole, a method to add disease resistance. They flourished beautiful strong healthy plants, which I watered and fed fortnightly with fish emulsion, seaweed concentrate and aspirin dissolved in water. I also covered their pretty little head with shade cloth to protect them from the suns rays.
So what went wrong? I figured the heatwave was just too much for them and so decided to pull them out. The real answer was glaringly obvious, staring right at me in a knobbly ugly knot of roots. My tomato plants were affected by Tomato root knot nematodes.
Root-knot nematodes (Meloidogyne spp.) are minute, worm-like animals which are very common in soil. They have a wide host range, causing problems in many annual and perennial crops. Tomatoes are among the most seriously affected, with the nematodes causing problems in all growing area Root-knot nematodes do not produce any specific above-ground symptoms. Affected plants have an unhealthy appearance and often show symptoms of stunting, wilting or yellowing.
Symptoms are particularly severe when plants are infected soon after planting. More commonly, however, nematode populations do not build up until late in the season and plants grow normally until they reach maturity. They then begin to wilt and die back with flowering, fruit set and fruit development being reduced.Below ground, the symptoms caused by root-knot nematodes are quite distinctive. Lumps or galls, ranging in size from 1 to 10 mm in diameter, develop all over the roots. In severe infestations, heavily galled roots may rot away, leaving a poor root system with a few large galls.
The common species of root-knot nematodes all have a wide host range, and most plants are able to host at least one species. Many important fruit, vegetable and ornamental crops are good hosts of these nematodes, they include:banana, cucurbits, grape, carnation, passionfruit, nectarine, capsicum beans, kiwi fruit, chrysanthemum, pineapple, tomato, carrot, eggplant, strawberry, rose, peach, celery, ginger, lettuce, papaw, pumpkin.
Nematode management
Crop rotation
Root-knot problems increase, and control becomes more difficult, when tomatoes or other susceptible crops are grown without rotation. Crop rotation will not eliminate infestations because root-knot nematodes can remain in the soil as eggs for at least a year between host crops, and most species can feed on a wide range of weeds.
However, rotation can significantly reduce losses when a field is again planted to a susceptible crop. Winter cereals are useful because they are generally poor hosts and little nematode reproduction occurs during the cold winter months. It is more difficult to find summer crops with good resistance to root-knot nematode but sorghum x Sudan grass hybrids (particularly cv. Jumbo) are useful against most populations of the nematode.
Crop rotation
Root-knot problems increase, and control becomes more difficult, when tomatoes or other susceptible crops are grown without rotation. Crop rotation will not eliminate infestations because root-knot nematodes can remain in the soil as eggs for at least a year between host crops, and most species can feed on a wide range of weeds.
However, rotation can significantly reduce losses when a field is again planted to a susceptible crop. Winter cereals are useful because they are generally poor hosts and little nematode reproduction occurs during the cold winter months. It is more difficult to find summer crops with good resistance to root-knot nematode but sorghum x Sudan grass hybrids (particularly cv. Jumbo) are useful against most populations of the nematode.
Fallow and cultivation
Repeated cultivation kills nematodes in the upper soil layers by exposing them to mechanical abrasion and the heating and drying action of the sun. If the field is maintained weed-free, nematodes also die of starvation. In warm, moist soils in Queensland, a 4 to 6 month fallow may reduce root-knot nematode populations by more that 95%. Longer periods of fallow are not normally economically feasible, and risk of soil erosion is increased.
Repeated cultivation kills nematodes in the upper soil layers by exposing them to mechanical abrasion and the heating and drying action of the sun. If the field is maintained weed-free, nematodes also die of starvation. In warm, moist soils in Queensland, a 4 to 6 month fallow may reduce root-knot nematode populations by more that 95%. Longer periods of fallow are not normally economically feasible, and risk of soil erosion is increased.
Sanitation
Nematode populations have the capacity to increase rapidly, so to prevent further multiplication, plants should be ploughed out as soon as the crop is harvested. At this time, most of the nematode population is in the roots rather than in the soil, so if these roots are removed from the field and destroyed (e.g. by burning), there is an immediate and substantial reduction in the nematode population.
Nematode populations have the capacity to increase rapidly, so to prevent further multiplication, plants should be ploughed out as soon as the crop is harvested. At this time, most of the nematode population is in the roots rather than in the soil, so if these roots are removed from the field and destroyed (e.g. by burning), there is an immediate and substantial reduction in the nematode population.
Organic matter and numbers of root-knot nematodes in tomato
Soil organic matter is known to be detrimental to nematodes. 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.
Timetable for decision-making on nematode management in tomatoes
Approx. time before planting date (months)
-12 months
Remove plants without shaking the soil from the roots. Destroy nematode-infested root systems Infested plants must be burned, or sealed in plastic bags and disposed of in the garbage.
Approx. time before planting date (months)
-12 months
Remove plants without shaking the soil from the roots. Destroy nematode-infested root systems Infested plants must be burned, or sealed in plastic bags and disposed of in the garbage.
-8 Months
Plough out crop immediately after harvest.
Maintain a weed-free fallow until a cover crop is planted.
Plant a cover crop that is not susceptible to root-knot nematodes, e.g. winter cereals or forage sorghum, mustard,
Plough out crop immediately after harvest.
Maintain a weed-free fallow until a cover crop is planted.
Plant a cover crop that is not susceptible to root-knot nematodes, e.g. winter cereals or forage sorghum, mustard,
-2 Months
Collect soil samples and either: do a bioassay; or test soil for nematodes.
0 Months
If the results of nematode analyses or bioassays, or the previous occurrence of nematode problems, suggest nematodes are likely to cause damage, either:
plant a nematode-resistant variety; or
apply a pre-plant nematicide. Where severe infestations occur, rest soil from growing host plants for three years. Control weeds assiduously.
Nemodode Information supplied by Tony Pattison, Department of Primary Industries and Fisheries, Queensland.
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