IPMP3.0, Oregon State University, Copyright 2000





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Northern root-knot nematode (Meloidogyne hapla) infests both peppermint and spearmint, but spearmint appears to be more tolerant (Eshtiaghi, 1975).

Life Cycle

Northern root-knot nematodes are typical sedentary endoparasites (see Fig. 3).

Symptoms and Effects on Plant Growth

Shoot weights of peppermint in greenhouse pots infested with a single egg mass (300 eggs) were reduced by 23% after four months. Root weight was increased, but roots were shorter, thicker and brittle. These effects increased slightly with greater inoculum densities (Eshtiaghi, 1975). Galling often increases root weight at the expense of shoots, but the heavier root mass is still less functional than in noninfected plants. While concentrations of N, P, Mn, and Al decreased in infected shoots, Ca increased. Spearmint shoot weights at four months were reduced by 27% when rhizomes were planted into pots with 100 J2 (= 265/quart soil). Larger inoculum levels did not decrease yields further. Root weights were depressed by 18%, and severe galling occurred. However, except for reduced size and weight of shoots, plant tops appeared normal. Concentrations of N, P, Mg, Mn, Fe and Al were decreased in shoots. Reduction in plant nutrients in shoots is due to reduced plant uptake and to diversion of nutrients from shoots to roots and sites of nematode infection. Thus, tops may look nutrient stressed at high infestation levels.

Northern root-knot nematodes may cause only slight damage the first growing season, but severe damage may occur in subsequent seasons as populations increase (Eshtiaghi, 1975). Cases of mint decline due to northern root-knot nematode have not been frequently documented. However, since few aboveground symptoms occur with infection by this nematode, it would be easy for this diagnosis to be missed unless roots are examined and/or nematode samples are taken. Eshtiaghi (1975) expressed concern that infection by northern root-knot nematode may suppress symptom expression by Verticillium. Masking infection by Verticillium may provide the opportunity for spread of this pathogen in symptomless rootstock. Presence of root-knot nematodes in peppermint is important to growers planning to rotate from mint into potato, since these nematodes will make tubers unmarketable if they are not managed properly.