Rotating land out of susceptible crops can be an effective and relatively inexpensive means for managing some diseases. To successfully use crop rotation for disease management, however, requires understanding the life cycle of the disease-causing organism (pathogen). Generally, the technique of using crop rotation for disease management is to grow non-host plants until the pathogen in the soil dies or its population is reduced to a level that will result in negligible crop damage. To manage a disease successfully with rotation, one needs to know (1) how long the pathogen can survive in the soil, (2) which additional plant species (including weeds and cover crops) it can infect or survive on, (3) other ways it can survive between susceptible crops, (4) how it can be spread or reintroduced into a field, and (5) methods for managing other pathogen sources. For example, a pathogen that can survive in the soil but can also disperse by wind may not be successfully managed by rotation if an infected planting occurs nearby or the spores can disperse long distances.
Importance of Scientific Names in Disease Management
When designing a rotational sequence for managing a particular disease, focus should be on the pathogen’s scientific name, because common names can be misleading. For example, powdery mildew, downy mildew, bacterial blight, and fusarium wilt are usually caused by different pathogens in different crops. On the other hand, white mold, which occurs in several crops, is caused by the same fungus that causes lettuce drop.
Knowing whether the pathogen exists as specialized strains that limit the host range is critical for designing disease-controlling rotations. For example, all grasses get anthracnose; however, host specialization was recently found to occur in the fungal pathogen causing this disease. Consequently, grass weeds do not play as important a role as alternate hosts for anthracnose in corn as was originally thought. Strains of some fungi are called formae speciales (f. sp.); others are called pathovar (pv.). These abbreviations occur in some of the pathogen scientific names listed in Appendix 3.
Appendix 3 lists sources of pathogen inoculum and recommended rotation periods for diseases of vegetable and field crops in the greater northeastern United States (including the mid-Atlantic region). The number of years needed to suppress a disease cannot be stated precisely for many diseases because of the impact of other factors and lack of extensive research, but general guidelines have been developed from research and farm observations, as well as knowledge of pathogen biology. While these periods are based on research and observations from conventional production systems, they are generally applicable to organic systems because pathogen biology doesn’t change. However, if the activity of beneficial soil microorganisms that suppress a pathogen is much higher in an organic field than in a conventional field, the required rotation period might be shorter. On the other hand, if more organic matter, such as an incorporated cover crop, is present in an organic system, those pathogens that can survive on decomposing organic matter may be more difficult to manage. Knowing which weeds can host a disease is important, as these weeds will need to be controlled during the rotation (see Appendices 3 and 5). Avoiding reintroduction of the pathogen when the crop is planted again is also critical. For example, infested seed, transplants, or soil on farm machinery can reintroduce a pathogen to a clean field.
The sections below describe the biological basis for managing plant diseases with crop rotation. First, several critical aspects of pathogen biology are discussed. These dictate whether rotation is a potentially viable option for managing a particular pathogen and the disease it causes. Second, how the characteristics of certain rotation crops affect pathogens is considered: Good rotations do more than simply provide an unsuitable host. Third, the impact of cover crops and incorporated green manures on diseases is considered. Fourth, various environmental and management factors that affect the success of crop rotation for disease suppression are discussed. Finally, some selected diseases that can be managed successfully with rotation are discussed, followed by some examples of diseases that cannot be controlled by rotation. These examples help explain the factors that affect rotation success. Although the focus here is on diseases of vegetable and field crops in the northeastern US, the principles are broadly applicable, and many of the specific diseases discussed occur in other regions as well. Because the same disease name is often applied to diseases caused by several different pathogens, scientific names are used frequently in the following sections to avoid ambiguity (see sidebar 3.1).
Pathogen Characteristics that Determine the Success of Rotation and Length of the Rotation Period
Rotation can effectively suppress a crop disease when the target pathogen is capable of surviving in the soil or on crop debris for no more than a few years. Some fungal and bacterial pathogens can survive in soil only in crop debris, and these are the most suitable pathogens to target for management with crop rotation because they cannot survive once the debris has decomposed. Pathogens that survive on soil organic matter but for only a few years can also be managed with crop rotation. These short-term residents of the soil are called soil invaders or soil transients. Pathogens in this group vary in the length of time they can survive, and thus in the length of rotation needed.
“Wind, irrigation water, or insects can spread the pathogen from infected crops and re-infest the field after rotation.”
Survival time partly reflects the type of plant host tissue infected. For example, the barley scald pathogen primarily infects leaves and leaf sheaths, which decompose fairly rapidly. In contrast, the net blotch pathogen also infects barley stems, including the nodes, which are more resistant to decay. Consequently, a longer rotation is needed to manage net blotch than to manage scald. Infected seed, and also wind-dispersed spores for the net blotch pathogen, are additional sources of these pathogens that need to be managed to ensure successful control through rotation.
Similarly, managing the bacterial canker disease that affects tomatoes requires a longer rotation than is needed to manage bacterial speck and bacterial spot. The canker-causing bacteria get inside tomato stems, whereas speck and spot are restricted to rapidly decomposing leaves and fruit. All three pathogens can be seed-borne. Rotation is only one aspect of a good control program. Managing other sources of bacterial pathogens is critical for success. This is covered in detail in the section on specific diseases below.
A few fungal and bacterial pathogens are true soil inhabitants, able to grow on organic matter in the soil. Such organisms are referred to as saprophytes. These are hard to manage with rotation. Examples of soil inhabitants are the fungi Pythium, Rhizoctonia, and Fusarium and the bacteria Erwinia, Rhizomonas, and Streptomyces.
Several species of Pythium and Rhizoctonia are commonly found in most soils as part of the normal soil flora. These fungi attack seeds and the roots and stems of tender seedlings, causing seed decay and damping-off. Pythium species also cause fruit rot in cucurbits, and Rhizoctonia solani causes wirestem, bottom rot, and head rot in crucifers. Although crop rotations will not completely control these fungi, reducing the pathogen population by rotating with small grains can reduce losses in subsequent crops. Since these saprophytes can also use fresh plant residues, incorporating large amounts of organic matter will stimulate their growth. Thus, crops planted too soon after incorporating a cover crop could be severely affected by these fungi. Other types of organic matter, such as leaves or incompletely decomposed compost, could have a similar effect. Some fungi that cause fusarium wilts can survive in or on roots of plants that do not develop symptoms (“symptomless carriers”), and they can also grow as saprophytes on plant debris and other partly decomposed organic matter.
Some fungal pathogens produce specialized structures that, like seeds, enable them to persist in a state of dormancy. These structures help the pathogen survive periods when host plants are absent, as well as cold winter temperatures and other adverse conditions. The maximum survival time varies among types of structures and species of pathogen. Fungal structures capable of dormancy include oospores, sclerotia, chlamydospores, and cleistothecia. Similarly, some pathogenic nematodes produce cysts. Recognizing that these terms refer to resting structures is helpful when reading about plant diseases because they indicate that a pathogen can potentially persist in soil.
Some pathogens are heterothallic, which means they produce the dormant structure only when individuals of opposite mating types (the fungal equivalent of male and female) interact. This is important because presence or absence of the different mating types can determine whether the pathogen persists in the soil. For example, although the cucurbit downy mildew fungus, Pseudoperonospora cubensis, is potentially capable of producing oospores, only one mating type occurs in the US; thus, it cannot produce oospores, and that prevents it from surviving winter in the northeast US. In contrast, the onion downy mildew fungus, Peronospora destructor, does produce oospores in the northeast US, and they can survive four to five years in soil. The situation can change, however: Until recently only one mating type of the late blight fungus, Phytophthora infestans, existed in the US. Now two mating types are present, and the pathogen can persist in soil as oospores. Phytophthora erythroseptica, which causes pink rot in potato, is homothallic. Thus, it can produce oospores when just one mating type is present.
Sclerotia and chlamydospores are structures that can be produced without interaction between fungi of opposite mating types. Sclerotia produced by Colletotrichum coccodes, which causes anthracnose and black dot in tomato, survive at least eight years. Those formed by Sclerotinia sclerotiorum, the white mold fungus, can survive up to ten years. Rhizoctonia spp. also produce sclerotia. Verticillium dahliae, which causes verticillium wilt, produces tiny sclerotia (microsclerotia) that can survive up to 13 years. Fungi causing fusarium wilts can persist for many years as chlamydospores.
The target pathogen should have a narrow host range for rotation to be successful. Peronospora farinose f. sp. spinacia causes downy mildew only in spinach. Another spinach pathogen, Albugo occidentalis, causes white rust in spinach and in some species of lambsquarters and goosefoot. However, the strains of fungi that attack the different species are specialists, and this host specificity can prevent cross infection. Thus, white rust may occur only on spinach or only on weeds in a field. In contrast, the fungus Sclerotinia sclerotiorum, which causes white mold, can infect more than 360 plant species. Corn and cereals are among the few non-host crops that can be used in rotation to decrease abundance of this pathogen. Weeds must be managed carefully, however, for this rotation to be successful. In addition, rotation out of susceptible crops for at least five years is needed because this fungus produces long-lived sclerotia. Effectiveness of a rotation can be compromised when nearby plantings have white mold. Wind, irrigation water, or insects can spread the pathogen from infected crops and re-infest the field after rotation.
When considering rotation to manage a fungal pathogen that produces wind-dispersed spores, it is critical to know how far the spores can travel. Powdery mildew and downy mildew fungi produce spores that can disperse great distances. These cucurbit pathogens can move up the entire eastern coast of the US each year, helped by the sequential planting of these crops from south Florida to Maine as conditions become favorable. Clearly, pathogens like these would be difficult to control with rotation. In contrast, downy mildew of onion can be controlled with rotation, partly because the crop is not grown as extensively. Several other fungal pathogens that attack leaves, including Alternaria, Cercospora, Septoria, and Stemphylium, are more effectively controlled by rotation because they produce large spores. They disperse only short distances by wind, although they can leave the field on equipment. Other fungal pathogens, such as Colletotrichum, produce spores on leaves and fruit that disperse by splashing water. Bacteria are also dispersed by splashing water and in windblown water droplets.
New findings or changes in the pathogen can affect rotation guidelines. For example, a short rotation was initially thought to be adequate for Phytophthora capsici, which causes blight in cucurbits, pepper, tomato, and eggplant. Recently this pathogen was found on new hosts (lima bean, snap bean, purslane, and a few other weeds), which may explain why short rotations have not been effective. Additionally, it may be able to move between fields more easily than anticipated, possibly accounting for its occurrence in fields where susceptible crops had not been grown. Recently, most cases of early blight in tomatoes have been shown to be caused by a different species of Alternaria than the species causing early blight in potato. Consequently, early blight may not be more severe in tomatoes that follow early blight–affected potatoes than in tomatoes that follow other crops.
As a group, plant-pathogenic nematodes are more difficult to manage with rotation than fungi and bacteria because almost all exist for part of their lives in the soil. Only a few nematodes attack just leaves, rarely entering soil, and none of these infect vegetable or grain crops.
None of the few soilborne viruses that affect vegetable or grain crops occur in the northeastern US. Most viruses cannot persist in soil between crops because they survive only in living host tissue or vectors, and few viruses are vectored by nematodes or soilborne fungi. However, some viruses can persist between crops in weeds—for example, cucumber mosaic in chickweed and potato virus Y in dandelion (Appendix 5).
Beneficial Plants to Include in Rotations
The typical focus in designing a rotation for disease management is to alternate among crops that are susceptible to different pathogens. Alternating crops in different families is a good starting point, but some pathogens attack crops in two or more families. For example, Phytophthora capsici causes blight in cucurbits, peppers, and lima beans. Corn, small grains, and other grasses are usually good crops to rotate with vegetable crops. Fusarium fruit rot, however, has been more common in pumpkin following corn.
Some plants suppress pathogens in addition to being unsuitable hosts. These include some cover and green manure crops, as well as cash crops. Including disease-suppressive species in a rotation sometimes reduces the time needed before a particular cash crop can again be produced successfully. Examples include some legumes and crucifers. These plants suppress pathogens by stimulating beneficial organisms in the soil and by producing toxic chemicals. The specific mechanisms involved appear to vary with the crop and the pathogen. Depending on the mechanism, the beneficial effect can disappear shortly after incorporation or last for years. Suppression can vary with how well the pathogen is established in a field. Also, to achieve success, beneficial crops may need to be grown more than once before a susceptible cash crop is replanted. It is important to remember that incorporating large quantities of biomass in the form of green manure stimulates general microbial activity, which can include pathogens like Pythium, as described earlier.
Including legumes such as clover, pea, bean, vetch, and lupine in crop rotations has been recognized as beneficial for disease management since ancient times. Legumes stimulate the growth and activity of soil microbes, in addition to increasing soil nitrogen and organic matter. Hairy vetch residue incorporated into soil reduces fusarium wilt in watermelon and enhances crop growth. On the other hand, hairy vetch is a good host for root-knot nematodes.
“The degree of control was better than that achieved by fumigating with chloropicrin, a product used by conventional farmers.”
Members of the mustard plant family (crucifers) release substances while decomposing that are toxic to some fungi, nematodes, and even weeds; and they also stimulate beneficial microorganisms. One group of chemical breakdown by-products from these plants is the volatile isothiocyanates. These originate from glucosinolates, which are themselves harmless. Glucosinolate content varies among plants of the mustard family. White mustard, brown mustard, and rapeseed have especially high concentrations. IdaGold is a yellow mustard variety bred for high glucosinolate content. Glucoraphanin is a glucosinolate found at much higher levels in broccoli than in other cruciferous plants. Using these plants to manage pests is called biofumigation. Research has been done in California with mustard seeded in fall and incorporated in spring. In the northeast US, mustard would be seeded in early spring, then incorporated several weeks later when it is in full flower and organic matter is at a maximum. Soil temperature should be 59° to 77°F. Amending the soil with crucifer seed meal can similarly suppress disease. Isothiocyanates are thought to be less damaging to beneficial soil organisms than conventional chemical fumigants. These compounds can also be toxic to crops; thus, planting should be delayed about a month after incorporation. Quantity of isothiocyanates produced can vary with soil type, as well as variety of crucifer. The degree of disease control has been related to the quantity of isothiocyanates in some systems, but in other systems disease suppression evidently is due to another mechanism. Beneficial microorganisms stimulated include myxobacteria and Streptomyces.
Note that although a mustard or legume crop can suppress some pathogens, such crops may also promote pathogens that attack plants in those families.
Examples of Specific Diseases Influenced By Crop Rotation
Carrot root dieback
The soilborne fungi Pythium spp. and Rhizoctonia solani can infect and kill the tip of carrot tap roots, causing them to become forked or stubby. In severe cases, they kill the plants. A study in California showed that when carrots were grown after alfalfa, populations of Pythium and Rhizoctonia were larger, and fewer marketable carrots were produced. The study also revealed more misshapen carrots and a higher population of Pythium when barley preceded carrots, but the barley residue may not have decomposed sufficiently before the carrots were planted. Carrots and pathogen populations were normal when onions or a fallow period preceded the carrot crop. Another reason not to grow alfalfa before carrots is that alfalfa is a host for the fungus causing cavity spot of carrot, Pythium violae.
“Rotation that includes a fallow period can be the key for controlling some pathogens that have a wide host range.”
The roots of mustard family crops that are attacked by the slime mold fungus Plasmodiophora brassicae become greatly swollen. This pathogen can survive in soil for seven years in the absence of mustard family crops or weeds. Clubroot has declined more quickly when tomato, cucumber, snap bean, and buckwheat were grown. Clubroot was effectively controlled by growing summer savory, peppermint, garden thyme, or other aromatic perennial herb crops for two or three consecutive years.
Rotating among crop families is generally recommended, because crops within a family are typically susceptible to the same diseases; however, growing broccoli immediately before cauliflower resulted in a reduction in verticillium wilt, even though these plants are closely related. Broccoli produces more of a specific glucosinolate and stimulates myxobacteria that reduce survival of Verticillium microsclerotia. The degree of control was better than that achieved by fumigating with chloropicrin, a product used by conventional farmers. Fresh broccoli residue is more effective than dry residue. The greatest reductions in pathogen microsclerotia occurred when soil temperatures were above 68°F. Verticillium wilt diseases of other crops in California were also suppressed by broccoli.
Verticillium wilt and scab of potato
Both these diseases were reduced when corn or alfalfa was grown the previous year rather than potato. Verticillium wilt severity also was lower when a buckwheat green manure preceded potato than when canola or a fallow preceded potato.
Lettuce drop and whitemold
Broccoli is also a good crop to grow in rotation with lettuce and crops susceptible to white mold. The number of sclerotia of the lettuce drop fungus, Sclerotinia sclerotiorum, decreased after residue from a spring broccoli crop was incorporated during the summer. This resulted in reduced incidence of lettuce drop in a fall lettuce crop. Similar results were obtained in California when two consecutive crops of broccoli in one year were followed by two consecutive crops of lettuce the next year. Density of sclerotia of Sclerotinia minor was lower following broccoli than where broccoli was not grown, and broccoli was associated with lower incidence of white mold in subsequent crops. In contrast, cover crops of Lana woollypod vetch, phacelia, and Austrian winter pea in California hosted S. minor, and incidence of drop was higher where lettuce was grown after these cover crops were incorporated than in plots that had been left fallow. This pathogen also caused disease on purple vetch but not on oilseed radish, barley, or fava bean. Oilseed radish, however, is a host of the clubroot pathogen and root-knot nematodes. Phacelia and purple vetch are also hosts of root-knot nematodes.
Other Factors Affecting Success of Rotation in Managing Disease
Rotation is more likely to be effective if the entire field is rotated out of susceptible crops rather than just the section previously planted to the crop. When farm equipment is used throughout the field, infested soil or crop debris can be moved from the contaminated section to other parts of the field. If a field is divided into management units, rotation can be effective within a unit if cultivators and other farm equipment are cleaned before working in another unit and water does not flow between units during heavy rainstorms.
“Rotations of at least five or seven years often prevent the pathogen population from building up to a level that can cause economic damage.”
Time required for rotation to be effective can vary with disease severity and environmental conditions. When a disease has been severe, a longer rotation may be needed to reduce the pathogen’s inoculum level sufficiently to avoid economic loss. A two-year period between wheat crops is generally needed to reduce Septoria leaf spot; however, just one year without wheat resulted in a similar reduction in disease severity when environmental conditions were less favorable for this disease. Sclerotia can be sensitive to drying. Some pathogens, such as Sclerotium rolfsii, which causes southern blight, are adapted to warm conditions and do not survive low soil temperatures. That is why southern blight does not occur in most of the northeastern US.
Using other cultural practices with rotation can be the key to successfully controlling some pathogens. For example, after rotating land out of tomatoes to reduce pathogen populations, staking and mulching the subsequent tomato crop minimizes the potential for pathogen propagules remaining in the soil to disperse up onto the tomato plants. For a similar reason, deeply burying infested crop debris and pathogen survival structures by moldboard plowing reduces disease incidence. For this to work, the residue must be buried deeply enough that it is not pulled back up during seedbed preparation and cultivation. Burying diseased material is especially useful against pathogens that produce sclerotia and those that infect only aboveground plant tissue. However, deep, full inversion plowing decreases soil health by burying beneficial organisms that live in the top few inches of the soil profile. The value of incorporating debris is illustrated by corn diseases, in particular gray leaf spot, that are more common and more severe under no-till production. Breaking up infested crop debris immediately after harvest—for example, with flail chopping or repeated disking—can hasten decomposition of the debris, thereby reducing survival time for those pathogens that cannot survive in soil without debris.
While several pathogens are more likely to cause disease in subsequent crops when infested crop debris is left on the soil surface, there are exceptions. An in-depth study on survival of Colletotrichum coccodes, which causes anthracnose and black dot in tomato, revealed that its sclerotia survive longer when buried shallowly in the soil than when on the soil surface, probably because temperature and moisture vary more on the surface. The sclerotia of this species also appear to survive longer when not associated with plant tissue, likely because the skin tissue of tomato fruits becomes colonized by beneficial fungi that can parasitize the sclerotia. Roots are an important, overlooked source of this pathogen. Tomato roots in several fields were found to be infected and to have sclerotia when there were no symptoms on aboveground parts of the plants. Additionally, this fungus is pathogenic on roots of other plants in the nightshade and cucurbit families, including several weeds (see Appendix 3). It can also survive on the roots of numerous other non-host plants (symptomless carriers), including chrysanthemum, white mustard, cress, cabbage, and lettuce. This could account for C. coccodes occurring on tomato roots in fields with no previous history of tomato or other crops in the nightshade family.
Rotation that includes a fallow period can be the key for controlling some pathogens that have a wide host range. Bacteria causing soft rot are generally not considered amenable to management by rotation because they are common soil inhabitants with a wide host range. However, one of the common bacteria causing soft rot, Erwinia, does not survive well in a field that is fallow and repeatedly tilled.
Some cultural practices can also negate the benefit of rotation. Incidence of scurf (Monilochaetes infuscans) in sweet potato can be increased when animal manure is applied. Diseases caused by the fungus Rhizoctonia solani can be enhanced when undecomposed crop residue is present at planting. Potato and onion cull piles can be sources of pathogen inoculum and thus need to be destroyed before planting the next crop. Tillage can spread disease through-out a field from an initial few infested areas.
Some Diseases That Can Be Managed with Crop Rotation
Understanding the mechanisms that allow or prevent management of specific diseases by rotation can improve success and avoid wasted effort. This section discusses some diseases that can be successfully managed by crop rotation; the next section discusses some diseases that cannot. Other diseases for which rotation is or is not effective are listed in Appendix 3.
Bacterial spot of pepper and tomato
The bacterium causing spot (Xanthomonas campestris pv. vesicatoria) can be effectively controlled with rotation because this pathogen cannot survive in the soil once diseased plant debris decomposes. A minimum of two years without a host crop is recommended.
Bacterial speck of tomato
This disease is more difficult to control with rotation than bacterial spot because the pathogen (Pseudomonas syringae pv. tomato) can survive on roots and leaves of taxonomically diverse weeds. Therefore, success requires good control of weeds and volunteer tomatoes during the rotation period. A study on survival of this bacterium showed that it lived up to 30 weeks on crop debris but less than 30 days just in soil.
Bacteria causing spot and speck, as well as Clavibacter michiganensis subs. michiganensis, which causes bacterial canker in tomato, can occur on seeds. Thus, subsequent crops should be planted with seed lots that have been tested and shown to have no detectable pathogen to avoid reintroducing the pathogen into the field. The seed should also be hot water treated, because bacteria could be present at a low, undetectable level. A description of how to treat seed with hot water is at https://vegetablemdonline.ppath.cornell.edu/NewsArticles/PepperLeafSpot.htm. These tomato diseases also affect peppers, but resistant pepper varieties are available.
Volunteer tomato and pepper plants could grow from infested seed left in the field from a previous crop. Thus, volunteers need to be destroyed during rotation to ensure successful suppression of bacterial diseases. Destroying volunteer crop plants is also important for other seedborne bacterial diseases, in particular, bacterial fruit blotch of watermelon. Bacterial pathogens of tomato can also survive on tomato stakes and planting supplies, so these materials need to be replaced or disinfected before reuse.
Northern root-knot nematode (Meloidogyne hapla) is the most common species of root-knot nematode in the northeastern US and the only one found during a recent study of vegetable soils in New York. The predominant species in the mid-Atlantic region is southern root-knot nematode (M. incognita), although northern root-knot nematode also occurs there. These nematodes have a large range of hosts that includes most vegetables. Growing sorghum, small grains, or grasses or leaving a clean summer fallow between crops can reduce the nematode population to a tolerable level. The effect of these crops on root-knot nematodes is short lived, so they should be routinely incorporated into the cropping sequence. Weeds need to be controlled during the rotation to grasses, because some, notably nutsedges, are good hosts for these nematodes. Varieties of alfalfa, common bean, soybean, cowpea, peppers, and tomato are available with resistance to certain root-knot nematode species.
Some commonly used cover crops, notably hairy vetch, are good hosts of root-knot nematodes.
Some Diseases That Cannot Be Easily Managed with Crop Rotation
Fusarium wilts of crucifers, cucurbits, pea, spinach and tomato
These diseases are difficult to manage with rotation because the pathogens can persist for many years in soil in the absence of their crop host. They persist as dormant chlamydospores and on roots of some non-host plants (symptomless carriers). Rotations of at least five or seven years often prevent the pathogen population from building up to a level that can cause economic damage. However, if the disease has been severe in a field, even seven years may not be enough. Selecting resistant varieties is a more effective and practical means of controlling fusarium wilt. Multiple races have been identified for many of its host-specific forms. Therefore, knowing what races have occurred in an area is important when selecting resistant varieties. Fusarium wilt fungi also can be seed-borne and are easily moved on infected transplants. They can also be easily moved between fields in soil on equipment. These are the major ways they are brought onto a farm. Drought, mechanical damage, low soil pH, soil compaction, and other stress factors can predispose plants to infection by fusarium wilt fungi. Fortunately, the fusarium wilts in various crops are caused by different strains of the fungus Fusarium oxysporum (see sidebar 3.1). Thus, for example, healthy muskmelons can be grown in a field where fusarium wilt previously affected watermelon.
When environmental conditions are favorable (warm and wet), Rhizoctonia solani, a common fungus in most soils worldwide, can become a serious pathogen on susceptible crops. This fungus is subdivided into several strains, which further complicates management. It has a wide host range. Potatoes, beans, lettuce, and cabbage are among the most important hosts. Other hosts include broccoli, kale, radish, turnip, carrot, cress, cucumber, eggplant, pepper, tomato, and sweet potato. Symptoms produced on a host can vary with the time of infection. For example, in crucifers it causes damping-off of seedlings, wirestem in young plants, bottom rot in midseason, and head rot as heads mature. Although difficult to manage, rotating away from the most susceptible crops for at least three years can be helpful; cereals are an especially good choice for rotation crops. Incorporating residue of host plants can help.
Crop rotation can be an effective and relatively inexpensive means to manage some diseases, but achieving success can be challenging, and some diseases cannot be managed with rotation. The challenge can be met by knowing which diseases can be managed with rotation and understanding the aspects of a pathogen’s biology that make it amenable to such control. Effectiveness can be improved by designing a rotation with crops that, in addition to not being a host, decrease pathogen survival by producing chemicals toxic to pathogens or stimulating beneficial organisms in the soil. Length of time needed in rotation to decrease disease occurrence may be shorter where beneficial organisms that affect the pathogen are more active. Understanding the reasons that some pathogens are not affected by rotation can allow a farmer to focus on more appropriate measures for managing these diseases, including use of resistant varieties and preventing introduction of the pathogen to a farm.
For further general reading on this topic, see References 20, 98 and 114; for further reading on field crops, see 3 and 118; for further reading on vegetables, see 4, 18, 21, 23, 43, 53, 66, 72, 94, 97, 105, 115 and 124.