Effects of the northern root-knot nematode (Meloidogyne hapla Chitwood, 1949) on Mitcham peppermint (Mentha piperita L.) and Scotch spearmint (Mentha cardiaca Baker) Public Deposited

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  • The northern root-knot nematode (Meloidogyne hapla Chitwood, 1949) is a widespread pest on many plants in temperate zones such as the Pacific Northwest (U. S. A. ) Peppermint and spearmint are two major agricultural crops in Oregon and Washington with a combined annual acreage of 50, 000 to 55, 000 and an oil value of about 25 million dollars. Both plants are attacked by a number of natural enemies including some species of plant parasitic nematodes with M. hapla considered to be most destructive. A pathogenicity study which included fifteen different isolates of M. hapla, revealed that peppermint was a more susceptible host plant than spearmint. These nematode populations were collected from four major mint growing areas of the United States (Idaho, Indiana, Oregon and Washington) with most populations obtained from Oregon and Washington. Usually no symptoms appear on the above ground parts of root-knot nematode infected plants. Therefore, it is possible that farmers in these areas may overlook infected mint planting stock and introduce the pest into new lands. Spread of the nematode could cause serious problems in mint and many other agricultural crops. Inoculation of peppermint cuttings and rhizomes with different densities of nematode egg masses caused varying degrees of root gall formation in three and four month growing periods. Regression slopes indicated that the relationships between inoculum densities and root gall formation was significant at the one percent level. However, inoculated rhizomes and the longer growing period caused more infection in plants. A gradual decline in plant vigor and productivity could be attributed to root-knot nematode infection which resulted in a significant shoot length reduction and reduced plant weights. Dry matter contents and concentrations of N, P, Mn, Fe and Al decreased, but there was a slight increase in Ca. Spearmint rhizomes inoculated with fresh second stage larvae of M. hapla produced equally severe galling in four months of growth regardless of the various initial inoculum densities. However, no significant relationships between inoculum densities and reduction in plant weights or shoot lengths developed in this host. Decreases in dry matter contents of spearmint plants as well as concentrations of chemical elements were similar to those of peppermint. A deterrent interaction between the fungus Verticillium dahliae Kleb. and M. hapla was observed when peppermint and spearmint plants were inoculated with the two pathogens. Significantly (at the one percent level) less root galls were formed when V. dahliae was combined with different densities of M. hapla larvae. Also interaction between fungus and nematode caused longer incubation periods and less severe Verticillium wilt disease symptoms. Weight reductions due to such interaction were insignificant and the data indicate that peppermint was more susceptible to the pathogens than spearmint. It can be concluded that M. hapla can infect both peppermint and spearmint without showing obvious disease symptoms on aerial parts of plants. Therefore, infections can remain unnoticed particularly in spearmint which has more tolerance to the nematode Introduction of root-knot nematode infected planting stock to new planting sites or other fields spreads the parasite which will affect other agricultural crops. Also suppression of Verticillium wilt disease symptoms by the nematodes could complicate or delay early diagnosis and control of wilt disease.
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