COVER CROP AFFECTS ON FOLLOWING POTATO CROPS

(In an article in the June issue of "The Grower", Dr. Alan McKeown covered soil management options for growers of early potatoes. Some findings of interest to ecological growers are included below.)

Some options suggested by McKeown included a cover crop, or replanting with a late vegetables crops such as cabbage or broccoli immediately after potato harvest (late June), or rotation with crops such as wheat. He cautions however, that certain cover crops can increase pest problems. His major considerations were root lesion nematodes and verticillium wilt. He adds that allelopathic or phytotoxic effects must also be considered.

Fall rye, while providing vigorous growth , can be difficult to kill, and is also a good host for nematode. One rotation which included rye-red clover had very high nematode populations (over 2000/kg. of soil). Sudan grass, a good choice for hot summer conditions, comes with conflicting reports on its impact on nematode levels. McKeown found that wheat, oats, and even rye (despite the above fact) had no effect on the yields of subsequent potato crops.

Red clover, a good source of nitrogen, unfortunately is also a good host for verticillium wilt. Barley consistently increases scab levels in following potato crops.

Winter canola and oil radish had nematode populations comparable with bare soil, indicating they suppress nematodes. All three of these options out-yielded a soybean cover crop, where higher nematode populations were found. Phytotoxicity (toxicity to living plant material) is a serious drawback to canola and oil radish however. This risk increases with greater maturity of the cover crop, and the cool, wet conditions of spring that favour the formation of phytotoxic substances.

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