Crop rotations, in general, provide numerous benefits to potato production, and serve multiple functions. They can help conserve, maintain, or replenish soil resources, including organic matter, nitrogen and other nutrient inputs, and physical and chemical properties. Crop rotations have been associated with increased soil fertility, increased soil tilth and aggregate stability, improved soil water management, and reduced erosion. Probably most importantly, for potatoes as well as many other crops, rotations are essential to maintain crop productivity and reduce the build-up of soilborne plant pathogens and diseases, which can devastate crops grown in multiple consecutive years. However, the different crops and rotation practices used vary considerably and can greatly affect the efficacy of crop rotations in enhancing potato production.
Research in New England
Over the last dozen years, researchers at the USDA-ARS, New England Plant, Soil and Water Laboratory, in Orono, Maine, led by plant pathologist Robert Larkin, have been examining the benefits of improved potato cropping systems through the use of green manures, cover crops, organic amendments, and disease-suppressive rotation crops for increased productivity, reduced disease, and enhanced sustainability of potato production in the northeast U.S.
Most crops that are not hosts to the same pathogens and pests as the primary crop are considered suitable rotation crops. For potatoes these complementary crops have traditionally been primarily small grains such as barley and oats, which result in disease reduction primarily by serving as a break in the host-pathogen cycle. However, there may be substantial differences among different crop types, species and cultivars in their effects on soilborne diseases and productivity.
Rotation crops that have specific and active mechanisms for reducing soilborne diseases are considered disease-suppressive crops, and can result in substantially greater reductions in disease than standard or non-suppressive rotation crops. For example, crops in the Brassicacae family (including canola, rapeseed, broccoli, cabbage, cauliflower, turnip, radish and mustards) are known to reduce disease through a process called biofumigation. When biofumigation is properly utilized, plant material is broken down in soil to release volatile toxins that can reduce soil populations of weeds, nematodes and pathogens, as well as cause changes in soil microbial communities.
In addition, how the rotation crops are managed can also greatly influence the subsequent effects on the potato crop. Rotation crops can be managed as full season harvested crops, thus providing an economic return; as full season or partial season cover crops, where there is no harvest and plant residues are left in place; or grown as a green manure, where all plant biomass is incorporated into the soil while still fresh and green, with each approach having different advantages and disadvantages. Biofumigation crops are primarily used as green manures in order to get the most out of their biofumigation potential. Incorporation of green manures also provides benefits in the form of increased organic matter and increased microbial biomass and activity.
In their earlier studies dealing with which rotation crops were best in two-year rotations with potatoes, Larkin and colleagues observed that canola and rapeseed rotations consistently reduced soilborne diseases such as black scurf and common scab, and increased yield relative to standard rotation crops over eight to 10 years of field trials. Also, addition of a winter rye fall cover crop to each rotation resulted in an additional reduction in disease and a slight yield increase.
However, all two-year rotations also resulted in increasing levels of Verticillium wilt and common scab over time, whereas three-year rotations were more stable. Subsequent studies have focused on various aspects of using disease-suppressive crops such as rapeseed, mustard blends, and sudangrass for the development of improved potato cropping systems by incorporating longer rotations, green manures, cover crops, and additional biological and/or organic amendments.
Keeping Out Disease
Use of organic amendments, such as compost, have consistently increased yield, but sometimes have also led to increases in soilborne diseases. One such study compared different cropping systems that were designed to achieve particular management goals of soil conservation, soil improvement, and disease suppression in three-year rotations, relative to a standard two-year barley rotation and non-rotation potato controls. The disease-suppressive system incorporated mustard blend and sudangrass green manures with additional fall cover crops, and it resulted in lower levels of soilborne diseases (black scurf and common scab) than all other systems (with decreases of 25 to 58 percent). Under irrigated conditions, the disease-suppressive system produced the highest overall yields (although comparable to the soil improvement system), with increases of 20 to 25 percent relative to the other rotations. The combined effect of the disease-suppressive rotation and irrigation resulted in a yield increase of 42 percent relative to the non-irrigated barley standard rotation when averaged over a study period of five years.
An overview summarizing more than a decade’s worth of field trials in Maine (more than 70 individual trials) involving Brassica rotation crops indicated that, although results varied by field and year, positive effects on yield and disease reduction have been observed in the majority of trials. Yield was significantly improved in more than half the trials, with increases averaging 10 percent and extending up to 38 percent (see table). Black scurf was significantly reduced in 70 percent of the trials, with average reductions of 42 percent and ranging up to 100 percent. Common scab was also reduced in many of the trials, with reductions up to 45 percent relative to standard rotations. Other diseases, such as powdery scab, silver scurf, and Verticillium wilt were also reduced in most trials where they occurred.
Recent studies have focused on how to best utilize these crops in potato production in the Northeast, determining which crops to use and effective management practices. In a direct comparison of five different rotation crops (mustard blend, rapeseed, sudangrass, barley and soybean) under four different management types (cover crop, harvested-residue not incorporated, harvested-residue incorporated, and green manure), crops managed as green manures produced overall lower disease (by 15 to 26 percent) and higher yields (by 6 to 13 percent). Crops harvested for seed then incorporated also provided significant benefits, but rotations managed strictly as cover crops were least effective. Mustard blend, sudangrass and rapeseed all reduced black scurf (by 16 to 27 percent) and increased yield (by 6 to 11 percent). Overall, the combination of mustard blend managed as a green manure was most effective, reducing scurf by 54 percent and increasing yield by 26 percent relative to a soybean cover crop.
Overall, based on their research, to produce the maximum benefit on soilborne diseases and tuber production, recommendations include going to a three-year or longer rotation, incorporation of a disease-suppressive rotation crop (such as a mustard blend), use of green manures and cover crops wherever possible, and increasing the overall diversity of rotation and cover crops used. However, although use of Brassica and other disease-suppressive rotation crops can substantially reduce soilborne disease problems, they cannot completely control them. Good rotation strategies should be used in conjunction with other crop and soil management approaches to achieve more sustainable production.