Crop Rotations for Managing Plant Disease

PP-705 (Revised,) October 1999
Marcia P. McMullen and H. Arthur Lamey, Extension Plant Pathologists

Crop rotation can help minimize plant disease potential by reducing populations of disease organisms surviving in the soil or on crop refuse. Although crop rotation reduces the risk of many row crop and cereal diseases, it does not eliminate them. Small amounts of the disease organism may persist in the soil or crop refuse over extended periods. In addition, crop rotation does not affect disease organisms that survive on or in the seed, such as the cereal smuts. Crop rotation also does not affect disease organisms that blow in from the south, such as the cereal rusts.

"Unfavorable" rotations do not automatically result in serious disease outbreaks, because disease development often is dependent on favorable weather. However, growing successive crops that are susceptible to the same disease organism greatly increases the odds of severe disease losses. When crop rotations are limited or short term, vigorous disease and pest monitoring is essential.

Some plant disease organisms are unique to a particular crop; others attack many crops. The presence of a particular disease within a crop has important implications for the next crop in that field. Common disease problems and rotations to help avoid them will be discussed.

Cereals

Rotation between cereal crops and broadleaf crops results in the least risk from plant diseases. Disease organisms that attack cereal crops usually do not attack broadleaf crops, and vice versa (Table 1).

Leafspot Diseases. Rotations among different small grains and corn result in less buildup of leafspot diseases than continuous cropping of the same crop. For example, planting wheat on wheat or barley on barley may result in buildup of several leafspot disease organisms such as tan spot of wheat and net blotch of barley (Table 1). One of the Septoria leafspots and the spot blotch fungus also build up when wheat and barley are rotated.

However, with a wheat/barley rotation there is much less likelihood of buildup of tan spot, the most serious wheat leafspot disease. This is because tan spot will not develop and survive on barley. The tan spot fungus will survive on wheat residue for several years, so a two year period between wheat crops will provide greater reduction in tan spot potential than only a oneyear rotation.

Rotation of oats with wheat or barley substantially reduces the risk of leafspot disease, because most organisms that cause leaf diseases of oats do not attack wheat or barley. An oatwheat or oatbarley rotation results only in a possible buildup of Fusarium root rot, which has not been a serious problem in North Dakota.

If short rotation intervals or no rotation intervals between wheat or barley are practiced, many leafspot diseases can be reduced through tillage practices and fungicide applications. The potential for early season leafspots can be reduced by complete incorporation of small grain crop residue into the soil several weeks before planting, which buries most leafspot disease organisms. However, later in the season leaf spotting can develop from spores blown in from neighboring fields. Rotational crops such as flax, oats, millet, mustard or canola may provide residue free of cereal leafspot pathogens for minimum or notill production of wheat or barley.

Fungicide applications for leafspot control may be beneficial if rotations are not ideal and crop residue is not buried (e.g. minimum till or notill). Fungicides may be planned as part of the production cost. However, sprays can be cancelled later if yield potentials at early flag leaf do not warrant the cost of fungicide application, or if the environment has not been suitable for disease development.

Bacterial diseases such as bacterial blight also survive in the soil and in stubble and straw of wheat, barley and oats. They are favored by continuous cropping, reduced tillage, sprinkler irrigation or long periods of wet weather. No fungicides are available for their control.

Common Root Rot. Wheat and barley are susceptible to the common root rot fungus, Cochliobolus sativus (Helmintho-sporium sativum). This fungus survives and multiplies in soil planted to wheat or barley. Planting nonhost crops in the rotation reduces the level of this disease organism. Nonhost crops include oats and all broadleaf crops. Fallow also will reduce the common root rot fungus population. Long term (three to four years) rotations will provide the best control. Deep tillage does not reduce the level of common root rot. Some reduction of common root rot severity has been seen under minimum and notillage practices. Good rotation is the best method for management of common root rot. Other management steps for common root rot include variety choice and fungicide seed treatment.

Ergot. Rotation between wheat and rye is not advisable — both crops are very susceptible to ergot. Wheat following rye may result in possible high levels of ergot in the wheat crop. Rye and Triticale also should not follow each other because of the danger of ergot. Many grasses, such as Agropyron (wheat grass), should not be rotated with wheat or rye because they also are frequently infected with ergot and because of a possible increase in leafspot diseases (Table 1).

Wheat Streak Mosaic. Wheat should not be planted into wheat stubble unless all volunteer wheat plants in that field are destroyed at least two weeks before planting. Destruction of volunteers reduces the danger of wheat streak mosaic, as wheat volunteers are potential reservoirs of the virus that causes the disease. Seeding wheat into stubble of other crops is preferred because of the reduced risk of volunteers. If possible, also avoid planting winter wheat next to green corn. Corn is commonly infected with wheat streak mosaic in southeastern North Dakota and can be a source of infection to adjacent winter wheat (Table 1). Barley and oats also are susceptible to wheat streak, but infection of these crops is not as commonly seen in North Dakota.

Takeall. Growing wheat and barley continuously for several years may result in a buildup of takeall root rot — potentially a very serious disease. The disease is most often found associated with fields under high soil moisture. Although takeall is more traditionally found associated with irrigated fields, the disease has been found in nonirrigated wheat fields in years of high rainfall and wet soils. Levels of takeall disease in a field can be reduced substantially if wheat is not grown for several years. Grassy weeds and volunteer wheat are frequent reservoirs of takeall and should be destroyed.

Scab (Fusarium head blight) of small grains and corn stalk and ear rot. Head scab of small grains is caused by several species of the fungus Fusarium. The scab fungi survive from year to year in small grain and corn residue. Wheat or barley planted into wheat residue are at higher risk of scab infection than when planted into fallow or nonhost crop residue. Scab has been seen in rye and oats in North Dakota, but infections are much less common than for wheat and barley.

Corn in rotation with small grains can result in the buildup of Fusarium species. These fungi also cause corn root rot and corn stalk and ear rot (Table 1). If corn and small grains are included in a rotation, they should be separated by other crops or fallow, if possible. The most serious scab problem is likely to arise if small grains are planted into corn stubble or cornstalk trash. The scab organism survives in corn residue and is readily available to infect the small grain head if favorable warm and wet conditions occur at the time of pollination. Irrigated grain planted after corn is a high risk rotation practice, because the continuous wet conditions are very favorable for scab infection. Land under irrigation with corn should be rotated to some other crop periodically.

Non-Cereals

Soilborne diseases

Sclerotinia (white mold). This fungus attacks many broadleaf crops (Table 1). Sunflowers are most susceptible. Dry beans, mustard, canola, lentils and safflower are highly susceptible when grown under irrigation or when the growing season is wet. Soybeans are less commonly attacked than dry beans, but Sclerotinia infections are fairly common if soybeans are solid seeded, the season is wet, or the crop is irrigated. Alfalfa, field peas, potatoes and garbanzo beans also are susceptible, but severe infections are less common than on highly susceptible crops. Infections in semileafless field peas, flax or buckwheat are rare in North Dakota.

Even low levels of Sclerotinia are sufficient to maintain a population of the fungus in the soil and cause problems for the next highly susceptible crop. Ideally, all susceptible crops should be in a fouryear or longer rotation, with no other highly susceptible crop in the rotation. This rotation interval is essential if there are high levels of Sclerotinia in a field. It may be necessary to avoid susceptible crops for five or six years on severely infested land. An example of a severe infestation would be a sunflower field that had over 10 percent Sclerotinia stalk rot. If Sclerotinia is not at high levels, a fouryear rotation is still desirable to prevent its buildup.

The best rotation crops for Sclerotinia control are small grains, grasses, corn, and sorghum, as these are not host crops for Sclerotinia. Many broadleaf weeds are susceptible, including wild mustard, marsh elder, lambsquarters, pigweed, and Canada thistle; they must be controlled when growing nonhost crops.

Sclerotinia attacks sunflower in two ways but usually attacks most other susceptible crops in only one way. Sclerotia (hard, black fungus bodies) survive many years in the soil and may germinate to infect sunflower roots, resulting in a wilt disease which is also called basal stalk rot. Wilt occurs whenever sunflower is planted on Sclerotiniainfested land.

When soil moisture is high for one to two weeks, the sclerotia form tiny mushroomlike structures (apothecia) that produce millions of airborne spores. These spores can infect the senescing flower parts of dry beans, canola, lentils, safflower, soybeans, field peas and garbanzo beans. They also can produce head rot and middle stalk rot in sunflower.

Spores for these infections may come from the same field or a nearby field that has sclerotia on or near the soil surface, regardless of the current crop being grown in that field. Thus, Sclerotiniafree fields may become infested as a result of airborne spore infections of susceptible crops or weeds; in some years this can be a very important means of spread. Favorable weather conditions for spore formation do not occur every year. Very favorable weather in the July months of 1993-1997 resulted in extremely severe white mold in dry beans.

Excellent rotations will not prevent Sclerotinia from occurring in a field if the disease organism is introduced into clean fields by planting infested seed. Sclerotia can become mixed with the seed at harvest. Planting certified seed reduces but does not eliminate the danger of introducing Sclerotinia into clean fields. North Dakota seed laws permit no more than one sclerotium per pound of sunflower seed.

Verticillium. Sunflowers should not be rotated with potatoes, since both crops are suceptible to this disease and both may allow populations of the fungus to increase (Table 1). Verticilliumtolerant sunflower hybrids can support a population increase of Verticillium that could affect a subsequent potato crop. Potato varieties differ in their susceptibility to Verticillium: Kennebec is very susceptible, Russet Burbank is susceptible and Reddale is resistant. Potato crops should be grown at least three years apart. Verticillium also can be tuberborne on potato, so diseasefree seed sources are important. Safflower also is susceptible to Verticillium, especially when grown under irrigation. Alfalfa is susceptible to Verticillium and should not be rotated with potatoes or sunflower unless separated by a minimum of three years.

Downy mildew of sunflower. This disease organism survives up to 14 years in the soil. Crop rotations are not suitable for its control, and currently available hybrids are not resistant to all races. Metalaxyl (Allegiance), Mefenoxam (Apron XL) or oxadixyl (Anchor) seed treatments have been used to protect against infection. Recently the downy mildew fungus developed resistance to metalaxyl, mefenoxam and oxadixyl. New seed treatments for sunflower downy mildew control are under development.

Rhizoctonia seedling blight and root rot.  The Rhizoctonia fungus is favored by warm, moist soils and causes seedling blight of sugarbeets, dry beans, soybeans, flax and many other crops (Table 1). Rhizoctonia may also infect seedlings when germination is delayed by cool, wet soils. It also causes a root rot of alfalfa, dry beans, soybeans and sugarbeets and black scurf of potato. However, no one strain of Rhizoctonia infects all of these crops. Some common strains in North Dakota are designated AG22, AG3, and AG4. AG22 infects sugar-beet and results in root and crown rot, and it also causes a root rot of dry bean and soybean; sometimes it also causes a seedling blight of sugarbeet. AG3 is primarily a pathogen of potato and causes black scurf. The AG3 strain that attacks potato is both soilborne and tuberborne, so diseasefree seed sources and crop rotation are important for potato culture. AG4 is not so selective; it can cause seed rot, seedling blight and occasionally root rot of sugarbeets, plus similar diseases in alfalfa, dry beans, soybeans and canola. Growers should avoid short rotations between sugarbeets and dry beans or soybeans. Crops susceptible to the same strain or strains of Rhizoctonia should not be grown more often than once every three years.

Aphanomyces seedling blight and root rot of sugarbeet. Aphanomyces is a severe disease of sugarbeet that occurs on wet or waterlogged soils during warm weather. Extremely wet conditions in some areas during the summers of 1993, 1997, and 1998 favored Aphanomyces, especially in southern Minnesota. The fungus produces spores which survive over 20 years in the soil, making it almost impossible to control by rotation. It also survives on pigweed, kochia and lambs-quarters. Infested fields may need a better drainage system to avoid severe Aphanomyces damage in warm wet years. Aphanomyces tolerant hybrids and seed pelleted with Tachigaren fungicide is the best way to manage the disease in severely infested fields.

Potato scab. The potato scab fungus is soilborne and survives many years in the soil. Scab is a common problem of garden potato but rather uncommon in commercial potatoes. If potatoes are planted in infested fields, the disease is best controlled by the use of resistant varieties. Do not use animal manure on fields where potatoes are grown in the rotation, as animal manure can increase the scab disease potential.

Sugarbeet nematode. The sugarbeet nematode has not been reported in the Red River Valley of Minnesota and North Dakota in recent years, but it has been reported in the Red River Valley of Manitoba.  Canola is a host of the sugarbeet nematode and should not be grown in rotation with sugarbeets (Table 1); if it is grown in rotation, it should be equated to sugarbeet in the rotation sequence for purposes of determining sugarbeet rotation intervals.

Row Crop Diseases Borne on Crop Residues

Certain row crop disease organisms are borne on crop residues. Examples are dry bean rust, safflower rust, sunflower rust, bacterial blight of dry beans, potato early blight and late blight, Cercospora leaf spot of sugarbeet, white rust of mustard, blackleg of canola, and Ascochyta blight
and arthracnose of lentil. A threeyear rotation is desirable for most of these crops. A fouryear rotation for canola has been shown to provide better management of blackleg; use of varieties with some resistance to blackleg also helps to manage the disease. Management of potato late blight includes crop rotation and also destruction of potato cull piles prior to emergence of the potato crop in the spring.

Susceptible crops should not be planted next to a field that was severely diseased the year before, as those disease organisms that produce spores can be airborne or splash dispersed from crop refuse and volunteer plants. If a susceptible crop must be planted next to a field that was severely diseased the year before, tillage should be used to bury the infected crop residue early, and all volunteers should be destroyed before planting. Alternaria blights can be serious diseases of several row crops, including safflower, sun-flower,dry beans and canola. Each of these crops is attacked by different species of Alternaria. Since the Alternaria fungi survive on crop residues, crop rotation may help reduce the severity of these diseases. For Alternaria blight of safflower, a threeyear rotation or greater should be used in eastern North Dakota, as well as an Alternaria tolerant variety.

Certain diseases borne on crop residues also are seedborne: safflower rust, bacterial blight of dry beans, blackleg of canola, and Ascochyta blights of garbanzo beans and lentils. If diseased seed is used, rotations alone will not control these diseases. Selection of clean seed sources is desirable whenever feasible. Seed treatment is recommended to protect against the seedling phase of safflower rust, seed transmission of canola blackleg, and seed transmission of Ascochyta on garbanzo beans and lentils.

Conclusions

Unrelated crops are excellent in a rotation for disease man-agement. Small grains, corn, sorghum and other members of the grass family should be routinely rotated with broadleaf row crops. When government programs, marketing problems, or environmental constraints make the best rotations unfeasible, then the least risk rotations should be used. The greatest disease risk is always assumed when no rotations are practiced, i.e. continuous cropping of the same crop. Good rotations reduce the amount of soil and residueborne overwintering pathogens that can start disease epidemics. Crop rotations, which provide many benefits in addition to disease control, always should be considered when cropping decisions are made.

Table 1. Some important disease organisms that are affected by crop rotation, and the crops they attack.^a

Click here for an Adobe Acrobat PDF version of this table. (7KB)

^a Most disease organisms that attack only one crop are not listed here.
Relative prevalence and severity is indicated as follows: +++ = common and severe;
++ = moderately common, moderately severe; + = occasional and/or not severe,
(+) = published reports indicate that the crop is susceptible, but the disease has not
been observed in North Dakota.

^b Several species of Septoria and Stagonospora cause leaf diseases in wheat and
barley. Some are specific for wheat, some are specific for barley, but Stagnospora
nodorum and Stagonospora avenae f. sp. triticea attack both wheat and barley.

^c Wheat includes hard red spring wheat, hard red winter wheat and durum wheat.

^d Seed rot only; no seedling disease or root rot.

^e May survive saprophytically or as a low level pathogen on roots and maintain a
population.

PP-705 (Revised,) October 1999