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CLEAN ROOTSTOCK |
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CONTENTS |
Regardless of what fumigant material is used, or even if fields are not
fumigated, new fields should be planted with rootstock which has been tested for nematodes
and Verticillium. The benefits from expensive soil fumigation may be totally lost
if the field is planted with infested rootstock. Endoparasites such as root-lesion and
root-knot nematodes can be easily transported inside roots and rhizomes and infest new
areas when these materials are planted. Ectoparasites are also easily transported in soil
adhering to planting material. Mint nematodes which are present on planting stocks can
increase to destructive levels within 2-4 years, requiring treatment or removal of the
stand (Pinkerton and Jensen, 1983). Inadvertent spreading of nematodes on infested
rootstock over the years may be the primary reason why the majority of mint fields are
infested. Furthermore, these nematodes remain to potentially damage other crops when the
field is rotated out of mint. However, while it is conceivable that mint nematodes are
transported from the Willamette Valley to Central Oregon, they probably do not become
established. Mint nematodes are very susceptible to freezing with less than 2% surviving
in frozen soil (30 F) for one week. (Konicek, 1961). This low tolerance to frozen soil
conditions may explain why mint nematodes are rarely recovered from Central Oregon. The variety of mint planted may also be important in maintaining stand vigor on nematode infested soils. Bergeson and Green (1979) reported that Black Mitcham was more tolerant to P. penetrans at the first cutting but that all cultivars had significant foliage reduction at the second harvest. Under greenhouse conditions with all other factors kept as uniform as possible, Murray Mitcham was the least tolerant of damage by P. penetrans (Pinkerton, 1983). Todd and Black had nearly equal tolerance in the greenhouse but, in the field, Todd expressed more damage than Black. Apparently Todd is more predisposed to nematode damage when other environmental stresses are added as well. However, while Black is the most tolerant of root-lesion nematode damage, it is the most susceptible variety to Verticillium wilt. Therefore in fields infested with Verticillium and root-lesion nematodes, Todd is the best cultivar of the three. Murray should not be planted in fields infested with P. penetrans, particularly on light textured soils. Existing fields of Murray with nematode damage may require higher rates or more frequent applications of Vydate to maintain stand vigor and yield. Varieties currently being developed by the mint industry should be tested for their tolerance to nematodes in the presence of other environmental stresses, including Verticillium wilt. |
