5 Ways for Management of Soil Borne Diseases in Plants

The following points highlight the five ways for management of soil borne diseases. The ways are: 1. Mycorrhizal Fungi 2. Crop Rotation 3. Plant Nutrients 4. Compost 5. Direct Inoculation with Beneficial Organisms.

Way # 1. Mycorrhizal Fungi:

Among the most beneficial root-inhabiting organisms, mycorrhizal fungi can cover plant roots, forming what is known as a fungal mat.

The mycorrhizal fungi protect plant roots from diseases in several ways:

1. By providing a physical barrier to the invading pathogen. A few examples of physical exclusion have been reported. Physical protection is more likely to exclude soil insects and nematodes than bacteria or fungi. However, some studies have shown that nematodes can penetrate the fungal mat.

2. By providing antagonistic chemicals. Mycorrhizal fungi can produce a variety of antibiotics and other toxins that act against pathogenic organisms.

3. By competing with the pathogen.

4. By increasing the nutrient-uptake ability of plant roots. For example, improved phosphorus uptake in the host plant has commonly been associated with mychorrhizal fungi. When plants are not deprived of nutrients, they are better able to tolerate or resist disease-causing organisms.

In field studies with eggplant, fruit numbers went from an average of 3.5 per plant to an average of 5.8 per plant when inoculated with Gigaspora margarita mycorrhizal fungi. Average fruit weight per plant went from 258 grams to 437 grams. A lower incidence of Verticillium wilt was also realized in the mycorrhizal plants.

Protection from the pathogen Fusarium oxysporum was shown in a field study using a cool-season annual grass and mycorrhizal fungi. In this study the disease was suppressed in mycorrhizae-colonized grass inoculated with the pathogen.

Way # 2. Crop Rotation:

Rotation to a non-susceptible crop can help break this cycle by reducing pathogen levels. To be effective, rotations must be carefully planned. Since diseases usually attack plants related to each other, it is helpful to group vegetable rotations by family e.g., nightshades, alliums, cole crops, cucurbits.

The susceptible crop, related plants, and alternate host plants for the disease must be kept out of the field during the rotation period. Since plant pathogens persist in the soil for different lengths of time, the length of the rotation will vary with the disease being managed. To effectively plan a crop rotation, it is essential to know what crops are affected by what disease organisms.

In most cases, crop rotation effectively controls those pathogens that survive in soil or on crop residue. Crop rotation will not help to control diseases that are wind-blown or insect vectored from outside the area nor that can survive long periods in the soil without a host like Fusarium,. Rotation, by itself, is only effective on pathogens that can overwinter in the field or be introduced on infected seeds or transplants.

The period of time between susceptible crops is highly variable, depending on the disease. For example, it takes seven years without any cruciferous crops for clubroot to dissipate. A three-year crop rotation is the standard recommendation for control of black rot (Ceratocystis fimbriata), stem rot (Fusarium oxysporum), and scurf (Monilochaetes infuscans) in sweet potatoes.

Way # 3. Plant Nutrients:

Soil pH, calcium level, nitrogen form, and the availability of nutrients can all play major roles in disease management. Adequate crop nutrition makes plants more tolerant or resistant to disease. Also, the nutrient status of the soil and the use of particular fertilizers and amendments can have significant impacts on the pathogen’s environment.

One of the most widely recognized associations between fertility management and a crop disease is the effect of soil pH on potato scab. Potato scab is more severe in soils with pH levels above 5.2. Below 5.2 the disease is generally suppressed.

Sulfur and ammonium sources of nitrogen acidify the soil, also reducing the incidence and severity of potato scab. Liming, on the other hand, increases disease severity. While lowering the pH is an effective strategy for potato scab, increasing soil pH or calcium levels may be beneficial for disease management in many other crops.

Adequate levels of calcium can reduce clubroot in crucifer crops (broccoli, cabbage, turnips, etc.). The disease is inhibited in neutral to slightly alkaline soils (pH 6.7 to 7.2). A direct correlation between adequate calcium levels, and/ or higher pH, and decreasing levels of Fusarium occurrence has been established for a number of crops, including tomatoes, cotton, melons, and several ornamentals.

Calcium has also been used to control soil-borne diseases caused by Pythium, such as damping off. Crops where this has proved effective include wheat, peanuts, peas, soybeans, peppers, sugarbeets, beans, tomatoes, onions, and snapdragons. Reduction of damping off in cucumbers after amending the soil with calcium is also helpful in controlling the disease.

Nitrate forms of nitrogen fertilizer may suppress Fusarium wilt of tomato, while the ammonia form increases disease severity. The nitrate form tends to make the root zone less acidic. Basically, the beneficial effects of high pH are lost by using acidifying ammonium nitrogen. Tomato studies have shown that use of nitrate nitrogen in soil with an already high pH results in even better wilt control.

Celery studies showed reduced Fusarium disease levels from using calcium nitrate as compared to ammonium nitrate. The nitrate nitrogen form also produced the lowest levels of Fusarium on chrysanthemums, king asters, and carnations.

Potassium fertility is also associated with disease management. Inadequate potash levels can lead to susceptibility to Verticillium wilt in cotton. High potassium levels also retard Fusarium in tomatoes. Severity of wilt in cotton was decreased by boosting potassium rates as well.

Phosphate can also be critical. Increasing phosphorus rates above the level needed to grow the crop can increase the severity of Fusarium wilt in cotton and muskmelon. In general, the combination of lime, nitrate nitrogen, and low phosphorus is effective in reducing the severity of Fusarium.

Way # 4. Compost:

Compost has been used effectively in the nursery industry, in high-value crops, and in potting soil mixtures for control of root rot diseases. To get more reliable results from compost, the compost itself needs to be stable and of consistent quality. Adding compost to soil may be viewed as one of a spectrum of techniques including cover cropping, crop rotations, mulching, and manuring that add organic matter to the soil.

The major difference between compost-amended soil and the other techniques is that organic matter in compost is already “digested.” Other techniques require the digestion to take place in the soil, which allows for both anaerobic and aerobic decomposition of organic matter.

Properly composted organic matter is digested chiefly through aerobic processes. These differences have important implications for soil and nutrient management, as well as plant health and pest management.

Rhizoctonia root rot caused severe infections, plant stunting, and premature death where no compost was applied. Plants growing under the sludge treatment suffered severe root infection. Compost is effective because it fosters a more diverse soil environment in which a myriad of soil organisms exist.

Compost acts as a food source and shelter for the antagonists that compete with plant pathogens, for those organisms that prey on and parasitize pathogens, and for those beneficials that produce antibiotics.

Root rots caused by Pythium and Phytophthora are generally suppressed by the high numbers and diversity of beneficial microbes found in the compost. Such beneficials prevent the germination of spores and infection of plants growing on the amended soil.

Systemic resistance is also induced in plants due to compost that activate disease resistance genes in plants. These disease resistance genes are typically “turned on” by the plant in response to the presence of a pathogen. These genes mobilize chemical defenses against the pathogen invasion, although often too late to avoid the disease.

Plants growing in compost, however, have these disease-prevention systems already running. Induced resistance is somewhat pathogen-specific, but it does allow an additional way to manage certain diseases through common farming practices.

Depending on feed stock, inoculum, and composting process, composts have different characteristics affecting disease management potential. For example, high carbon to nitrogen ratio (C: N), tree bark compost generally works well to suppress Fusarium wilts.

With lower C: N ratio composts, Fusarium wilts may become more severe as a result of the excess nitrogen, which favors Fusarium. Compost from sewage sludge typically has a low C: N ratio.

Some of the beneficial microorganisms that re-inhabit compost from the outside edges after heating has subsided include several bacteria (Bacillus species, Flavobacterium balustinum, and various Pseudomanas species) and several fungi (Streptomyces, Penicillin, Trichoderma, and Gliocladium virens).

The moisture content following peak heating of a compost is critical to the range of organisms inhabiting the finished compost. Dry composts with less than 34% moisture are likely to be colonized by fungi and, therefore, are conducive to Pythium diseases.

Compost with at least 40 to 50% moisture will be colonized by both bacteria and fungi and will be disease suppressive. Water is typically added during the composting process to avoid a dry ‘condition. Compost pH below 5.0 inhibits bacterial bio- control agents.

Three approaches can be used to increase the suppressiveness of compost. First, curing the compost for four months or more; second, incorporating the compost in the field soil several months before planting; and third, inoculating the compost with specific biocontrol agents.

Two of the more common beneficials used to inoculate compost are strains of Trichoderma and Flavobacterium, added to suppress Rhizoctonia solani. Trichoderma harzianum acts against a broad range of soil-borne fungal crop pathogens, including R. solani, by production of anti-fungal exudates.

The key to disease suppression in compost is the level of decomposition. As the compost matures, it becomes more suppressive. Readily available carbon compounds found in low- quality, immature compost can support Pythium and Rhizoctonia. As these compounds are reduced during the complete composting process, saprophytic growth of these pathogens is dramatically slowed.

Beneficials such as Trichoderma hamatum and T. harzianum, unable to suppress Rhizoctonia in immature composts, are extremely effective when introduced into mature composts. For Pythium suppression, there is a direct correlation between general microbial activity, the amount of microbial bio-mass, and the degree of suppression.

Pythium is a nutrient-dependent pathogen with the ability to colonize fresh plant residue, especially in soil that has been fumigated to kill all soil life.

The severity of diseases caused by Pythium and R. solani relates less to the inoculum density than to the amount of saprophytic growth the pathogen achieves before infection. Consequently, soils that are antagonistic to saprophytic growth of Pythium such as soils amended with fully decomposed compost—will lower disease levels. Rhizoctonia is a highly competitive fungus that colonizes fresh organic matter.

Its ability to colonize decomposed organic matter is decreased or non- existent. Like immature compost, raw manure is conducive to diseases at first and becomes suppressive after decomposition. In other words, organic amendments supporting high biological activity (i.e., decomposition) are suppressive of plant-root diseases, while raw organic matter will often favor colonization by pathogens.

Way # 5. Direct Inoculation with Beneficial Organisms:

There are a number of commercial products containing beneficial, disease-suppressive organisms. These products are applied in various ways including seed treatments, compost inoculants, soil inoculants, and soil drenches. Among the beneficial organisms available are Trichoderma, Flavobacterium, Streptomycetes, Gliocladium spp., Bacillus spp., Pseudomonas spp.

Currently, the role of BCAs (Biological Control Agents) is a well established fact and has become increasingly crucial, and in several cases, complementary or even replacing the chemical.

Counter parts where antagonistic fungi play an important part. Fungal based BCAs have gained wide acceptance next to bacteria (mainly, Bacillus thuringiensis), primarily because of their broader spectrum in terms of disease control and yield. In this context, Trichoderma spp. has been the cynosure of many researchers who have been contributing to biological control pursuit through use of fungi.

Fungi of the genus Trichoderma and Gliocladium are important bio-control agents (BCAs) of several soil borne phyto-pathogens. The molecular characterization of several wild isolates has shown a certain degree of polymorphism and the presence of three different ITS lengths and the secondary metabolites involvement in biocontrol has been recently reviewed.

Trichoderma use different mechanisms for the control of phyto-pathogens which include myco-parasitism, competition for space and nutrients, secretion of antibiotics and fungal cell wall degrading enzymes. In addition, Trichoderma could have a stimulatory effect on plant growth as a result of modification of soil conditions.

Trichoderma harzianum is an efficient bio-control agent that is commercially produced to prevent development of several soil born pathogenic fungi. Different mechanisms have been suggested as being responsible for their biocontrol activity, which include competition for space and nutrients, secretion of chitinolytic enzymes, mycoparasitism and production of inhibitory compounds.