Plant Pathology
Factsheet

Based on three years of study, planting SCN-susceptible soybeans no-till into wheat stubble (no-till, doublecrop soybeans) reduces the level of SCN at the end of the season by 60-75%. A light discing of the wheat stubble, prior to planting soybeans, decreases the residue effect on SCN, but end-of-season levels are still below those associated with fields lacking wheat residue. The reduction of SCN in doublecrop systems is due to the wheat residue and it's distribution on and in the soil - not to tillage or planting date. Planting date (full season vs. doublecrop) does not effect SCN in Kentucky.

WHAT THE RESIDUE EFFECT WILL DO

By significantly reducing end-of- season levels of SCN, the residue effect will provide soybean producers with more flexible production options. For example, in the absence of the residue effect, SCN levels will increase greatly on a susceptible soybean in a single growing season. Generally, the levels are such that SCN-susceptible varieties cannot be grown more frequently than once every 4 years. Non-host crops, such as corn, and/or soybeans resistant to SCN must be grown in the interim years. Three years in one or the other of these crops is generally needed to negate the SCN increase that occurs in a single year of a susceptible soybean. The value of the residue effect is in reducing the period necessary between susceptible crops from 3 years to 2 years. On some farms, it may be possible to grow susceptible soybeans every other year.

Growing susceptible soybeans once every 4 years is a manageable situation for most producers, but it is still somewhat restrictive. Producers may desire to grow susceptible soybeans more often than once every 4 years. Also, the interim 3 years may be a problem for some producers. Specifically, producers who are not comfortable with resistant soybeans may have to grow corn in consecutive years, which is not desirable. Alternatively, those lacking sufficient corn base may be forced to grow resistant soybeans more frequently than is recommended. Use of resistant soybeans more often than once every 3 years is not recommended because it encourages SCN race shifts. Race shifts, in turn, limit or eliminate the usefulness of resistant soybeans in SCN management programs. Race shifts are discouraged by growing susceptible soybeans as often as possible. The above problems will be moderated somewhat by the residue effect.

WHAT THE RESIDUE EFFECT WILL NOT DO

Yield loss in soybeans, due to SCN, is determined by the level of SCN that is present in the first 6-8 weeks following planting. The residue effect does not occur until late in the season, after SCN damage has already occurred. Thus, the residue effect is a valuable tool for overall SCN management, but it WILL NOT protect soybeans from damage if SCN levels are high at planting. Non- host crops or resistant soybeans must be grown in these cases.