Causal agent(s) and transmission
Currently the genus Meloidogyne comprises more than 90 described species and most of these are obligate parasites on plant roots and below-ground plant organs. Six of them are major agricultural pests and known to damage potato crops by reducing yield and affecting tuber quality with external deformations (gall) and internal flesh necroses. These root-knot nematodes (RKN) are obligate sedentary endoparasites which complete almost all their life cycle within the host. Besides the vermiform adult male, only the larval stage (J2) is motile and is the infective stage. The feeding activity within the potato tissues generates cell hypertrophy, leading to root and tuber deformations known as galls. These nematodes are extremely polyphagous and are able to multiply in numerous crops (sugar beetroot, carrot, cereals, rape, etc.) and weeds (nightshade, etc.).
- Diversity and distribution
Some species of Meloidogyne (RKN) are very common in hot climates, from the Mediterranean basin to the tropics: M. arenaria, M. javanica and M. incognita.
The observed damage on potato crops is not species specific and consists of root and tuber galls.
M. hapla, M. chitwoodi and M. fallax are species which are more adapted to temperate conditions and are present in Northern countries such as the Northern US states where M. chitwoodi was first described and Northern European countries where M. fallax was initially found as a deviating M. chitwoodi population.
Meloidogyne species are very serious worldwide agronomical pests. For M. chitwoodi and M. fallax that have only recently been identified in Europe, records of occurrence have just been initiated. According to European legislation, both species are classified as quarantine parasites and are therefore subject to mandatory control measures.
Symptoms on tubers
During the growing cycle, plants severely infected by Meloidogyne spp. may by stunted, turn yellow, wilt or die if affected by heat and lack of water.
Because of the nematodes’ feeding habit, symptoms occur on roots and tubers at harvest.
In roots, galls (photo 1) form, usually of irregular size, as an obvious proliferation of root tissues. Galls keep their whitish colour unlike powdery scab galls, which are initially white at harvest and then turn black after oxidation.
Easier to see are the symptoms on tubers. Both species cause characteristic blisters or galls on the surface of infected tubers (photo 2). These can be confused with common scab pustules (photo 3). When cutting the tuber across the galls, small white (gelatinous and translucent) masses are visible in the flesh (cortex), these are the mature pear-shaped female nematodes with their egg masses. Over time, these spots turn into dark lesions (photo 4).
Tuber infection by M. chitwoodi may subsequently develop into internal reddish spots around the female nematodes (photo 5).
Species identification requires laboratory expertise. Recent molecular tools have contributed to an easier and more reliable identification of these closely related species.
Once introduced into a field in any of the European agricultural areas, M. fallax and M. chitwoodi, populations will establish themselves almost anywhere where the potato crop is able to grow. Intensive cultural practices like absence of rotation or host crops in the rotation (carrot, black salsify), poor control of groundkeepers and use of farm-saved seeds are favourable conditions for their persistent establishment.
Special attention must also be paid to soil movement (or field exchange) and to the management of crop residues and waste from processing factories because of the potential dispersal of these root-knot nematodes.
Control measures are difficult because of the very wide host range of these nematodes. Moreover, accurate species identification is critical because of their distinct host ranges.
Different measures and factors have an effect on the control of the root-knot nematodes:
- the use of certified seed tubers is highly advisable as well as avoiding the introduction of soil, rooted plants or seed tubers originating from infected fields or areas;
- because no commercial potato cultivar is fully resistant to root-knot nematodes, nematicide treatment is usually the most effective measure in infested areas but, due to the high toxicity of nematicides, chemical control is now severely restricted by many authorities;
- in the case of low infestation, early planting and early harvest may limit tuber damage, as the disease develops mainly in warm conditions (late autumn);
- black fallow, i.e. sanitary period with no crop at all, reduces soil infestation with root
knot nematodes more safely than crop rotation which relies on the Meloidogyne species and races present in the field. Rotation with cereal crops is generally recommended in the presence of M. arenaria, M. javanica or M. incognita but not in the case of M. chitwoodi. Other crops like some Brassicaceae have some level of resistance to M. chitwoodi and can be used as green manure crops and may reduce nematode populations;
- for horticultural production, other crops and vegetables are resistant to M. arenaria, M. javanica and M. incognita (i.e. tomato cultivars carrying the Mi resistance gene) and can be successfully used in the rotation;
- alternative methods have been successfully tested under experimental conditions such as biological control using the predator fungus (Arthrobotrys) or the bacterium (Bacillus penetrans) but their application in the field still requires further research.