How can crop rotation be used to manage the soybean cyst nematode?
Crop rotation is used not only for SCN management, but also to benefit general crop management. Although SCN has a wide range of host plant species, only a few crops are its hosts (Table 5). Many crops, including alfalfa, barley, corn, oat, potato, sorghum, sugar beet, sunflower and wheat are not hosts for SCN and could be included in a crop rotation to reduce SCN population densities (Table 6).
Table 5. Hosts of soybean cyst nematode.
|Crop plants||Weed plants|
|Common and hairy vetch||Common chickweed|
|Soybean||Milk and wood vetch|
|Sweet clover||Mouse-ear chickweed|
|White and yellow lupine||Wild mustard|
Table 6. Poor hosts and nonhosts for SCN management rotation.
|Barrel medic||Flax||Sugar beet|
|Brassica cabbages||Marigolds||Sunn hemp|
|Corn||Red clover||White clover|
The number of years of nonhost crops needed to effectively lower SCN population density depends on many factors, including initial egg density, and soil biotic and abiotic factors that affect nematode mortality. In Minnesota, SCN survives well during winter, and with high populations after a susceptible soybean it may take as long as 5 years, depending on initial egg population density and soil environments, of nonhost or poor-host crops to reduce the SCN population prior to planting below a density (e.g., ~200 eggs/100cc of soil) that will not damage a susceptible variety (Fig. 12).
Fig. 15. The effect of two soybean cyst nematode population densities on the resistant soybean variety 'Freeborn.' Plant growth was significantly suppressed in the two left rows where SCN egg population densities prior to planting were 35,500 eggs/100cc of soil. The two rows on the right were planted into soil with 3,500 eggs/100cc of soil.
Some leguminous crops such as pea, sunn hemp, and Illinois bundleflower are poor hosts that produce SCN hatch stimulants and are more effective in lowering SCN population density than monocots including corn and wheat. Some crops such as marigolds and sunn hemp may produce compounds that have nematicidal effects.
In most cases in Minnesota where soybean is frequently grown, the short period of rotation with nonhost crops is not long enough to lower the egg population densities below levels that cause yield loss, and resistant varieties must be used to reduce yield loss.
Resistant varieties should be used at egg counts between 200-10,000 eggs/100cc of soil. SCN at a high density (> 10,000 eggs/100cc of soil) can cause a significant yield loss (> 2 bu/a) even to a resistant variety (Fig. 15). Consequently, fields with SCN population densities at or above 10,000 eggs/100cc of soil should be planted to a nonhost crop for one or more years until the population densities drop below that level. If the egg number is reduced sufficiently by the rotation of nonhosts and resistant varieties, a susceptible soybean can be used.
In some fields, because the soil is suppressive to SCN, 3 years of SCN-resistant soybean and nonhost (Fig. 12, brown arrow) may be sufficient to reduce the SCN population to a low level, and susceptible soybean can be considered. However, SCN population density should be determined before planting an SCN-susceptible soybean.
If nonhost crops are planted in rotation with soybean for sufficiently long periods such as in many organic-farming fields, SCN populations can be lowered to less than damaging levels (< 200 eggs/100cc of soil). At low SCN population densities, susceptible varieties can be considered to help avoid or slow down the development of SCN populations that may overcome resistance.