Three factors are needed to produce a plant disease: a susceptible variety or host, a favourable environment, and a virulent pathogen. These three elements constitute a plant disease triangle and a disease develops only when all three factors are favourable. The best way to reduce the risk of plant disease in zero tillage is to eliminate one of them. In most cases this involves using management practices that reduce the pathogen carryover. Good crop rotation is the most powerful way to reduce plant diseases.

ZERO TILL CONSIDERATIONS

Techniques to control plant disease are similar regardless of tillage system. However, a few aspects of zero till make adopting these techniques even more important.

Crop residues on the soil surface may influence the incidence and severity of plant diseases. Residues have little influence on diseases such as wheat stem or leaf rust which can come from spores carried long distances in the wind and not from the stubble. Unfortunately, many plant pathogens that cause leaf diseases, stem infections, and root rots can overwinter and survive on the crop residue left on the soil surface.

DECOMPOSITION IS ESSENTIAL

Survival of residue-borne diseases is closely correlated with residue decomposition. Tillage promotes residue decomposition by fracturing the residue. This increases access by residue-decomposing microorganisms. In comparison with conditions at the surface, the soil environment is better for sustaining decomposition because the soil microorganisms are protected from drying out.

Populations of Fusarium graminearum, the fungus which causes crown rot and head blight in wheat (scab), decline rapidly with tillage.1 However, in zero tillage it may take two or more non-scab years before it is significantly reduced.

Conditions in the root zone of zero till fields protect crops from drought and heat stress and, consequently, from some diseases.2 Root rot in cereals and peas has been less severe under zero till.3

The microenvironment of the soil and duff layer will affect the types of diseases present, the dynamics of the pathogens, and the risks associated with growing similar crops in close rotation (Table 1).

Burning crop residue is not recommended for controlling diseases in zero till and is a poor environmental practice. Burning will also not be hot enough to eliminate root stubble on which some pathogens can survive in the field. The buildup of cereal diseases can be predicted. However, we still have a lot to learn about diseases in oilseed and pulse crops in zero till.

DISEASES CAN BE REDUCED BY:

• using good crop rotation
• using tolerant or disease resistant varieties
• using pathogen-free seed with high germination
• eliminating volunteer plants that can harbour diseases
• planting at recommended seeding times and rates
• using proper fertilizer balances, especially nitrogen
• monitoring your fields and making strategic use of fungicides.

CROP ROTATION

Appropriate crop rotations lengthen the time between crop types so pathogen populations have time to decline. Crop rotations take advantage of the fact that plant pathogens important on one crop may not cause problems on another crop. For example, common disease problems on cereals will not affect oilseed, peas, or bean crops, and vice versa. Thus, by rotating among crop types, pathogens on the residue from previous crops in the field will not cause a disease problem on the crop being grown. Advancing the art of zero tillage by developing appropriate crop rotations is discussed in component 3 .

Soil and residue borne diseases

For persistant diseases, a long period between susceptible crops would be the best solution, but it is neither easy nor practical for most growers. Rotation is critical for some diseases, but for others it is less important. The level of risk depends on the shortest interval between crops with similar diseases. Diverse rotations reduce the risk of catastrophic losses caused by diseases, but flexibility is needed to take advantage of market opportunities.

Disease severity and yield losses are higher with crop monoculture than with more diverse rotations. For example, compared to continuous wheat production, the following four diverse rotations yielded 11-28% more grain and reduced root and leaf diseases of wheat.4

• wheat-canola-wheat-lentil
• wheat-peas-wheat-lentil
• wheat-sunola-canaryseed-lentil
• wheat-flax-winterwheat-peas

Yield losses of up to 20% resulted from tan spot and septoria blotch in continuous wheat crops, compared to when wheat followed non-cereal crops.5

Sclerotinia

Sclerotinia stem rot has a wide host range including canola, mustard, lentils, sunflower, potato, sweet clover, dry beans, buckwheat, and faba beans. It does not affect cereals and grasses, and has a limited ability to affect alfalfa. Shortened rotations with susceptible crops greatly increases the risk of this disease.

Sclerotinia stem rot of canola has increased by 39% when grown after a rotation with peas and beans compared to a rotation with potatoes and beans.6 The use of peas in a rotation increases sclerotinia stem rot in succeeding susceptible crops. This disease is difficult to detect in pea crops and may not cause reductions in pea yield. However, numerous sclerotia (small hard resting bodies of the fungus) are released to the soil during harvest. Sunola is another crop that can contribute numerous sclerotia to the soil. Flax is a better alternative for rotations. A more open canopy lowers the risk of airborne infection and subsequent development of sclerotia.

Leaf diseases

Similar crops types, like wheat, barley, other cereals and grasses usually have similar diseases. A short break of 1-2 years usually gives adequate management of cereal diseases.7 Fungal spores causing tan spot of wheat may be recovered from weathered straw after two years, but their recovery is greatly reduced compared to spore production on straw the season after harvest.8 A rotation interval of one year between wheat crops is sufficient to lower disease levels of septoria leaf blotch when conditions do not favour the disease. A longer interval is needed when conditions are favourable for the disease.9

In general, if cereals are planted more than two years in a row, do not grow the same type of cereal crop each year. Use a range of cereals such as wheat, winter wheat, barley, triticale, oats, and rye. Wheat and barley have more similar disease problems than oats and rye. Grasses and canary seed carry diseases similar to cereals. Growing wheat or barley after grasses or alfalfa-grass mixtures may increase the risk of root diseases.

Root diseases

Crop rotations are an effective way of reducing root diseases such as common root rot. Following 0 and 5 years of nonsusceptible crops, common root rot severity declined from 28 to 13%, respectively.10The decline of common root rot inoculum in soil is slow, and a long rotation gives effective control. To lower inoculum levels and reduce the risk of root diseases, almost any annual oilseed, pulse crop, or perennial forage legume can be used. Cereals, canary seed, and wheatgrass forage species should not be used as the fungus sporulates on the crowns of these crops and inoculum levels will therefore be maintained or increased.

Blackleg

For some diseases, the rotation interval is a critical factor for the long-term success of plant disease management. Virulent blackleg is widespread on canola in the northern plains. Losses of over 50% may occur in severely infected crops. Crop rotation is recommended for control of blackleg, although it may not always succeed since airborne spores can travel up to 8 km (5 mi.). However, the largest numbers of spores are produced on two year or older residues.11 Therefore, the residue from the canola crop grown in year 1 of a rotation will start producing the largest number of spores in years 3-4, so the risk of blackleg infection will be highest in those years from inoculum coming from older stubble.

Ascochyta blight

Unfortunately, crop rotation may have less impact with some diseases like ascochyta blight of peas. Survival of this pathogen is reduced when pea residue is buried, but it can survive for long periods as a saprophyte in soil. In fact, Ascochyta pinodes, which causes the disease, has been isolated from soil 20 years after the last pea crop. This pathogen also produces airborne spores that can easily move to adjacent fields. Consequently, growers in areas where peas are commonly grown should expect to see this disease irrespective of rotation or tillage.

Rotations will, however, reduce powdery mildew and other diseases of peas making a healthier crop to fight off aschochyta blight.

A long-term investment

Crop selection for minimizing disease risks should be thought of as a long-term investment. A rotation should be comprised of 50% grass species, including winter cereals, with the remainder divided among pulse, flax, and other oilseed crops. The shortest rotation interval would be 2 years, but 4-6 gives better disease control. Another way to reduce diseases is to extend the rotation more by including a cycle of forage production. Disease risk levels vary with different crop rotations (Table 2).

INTEGRATED PEST MANAGEMENT (IPM)

IPM is the combined use of biological, cultural, mechanical and chemical techniques to reduce pest risk. These techniques include: using resistant cultivars, appropriate crop rotations, using a trap crop, considering economic thresholds, banded pesticides, targeted application of crop protection products and fertilizers, use of biological control for insect pests, and the protection of natural predators. Field scouting helps make an IPM program successful. It gives early identification of pest problems, determination of economic thresholds, and evaluation of the usefulness of a management strategy.

Using IPM has many advantages. It increases management choices and gives the most cost effective strategies. IPM advocates using pesticides only when needed and delays the development of resistance. It improves the timing and efficiency of crop protection and prevents unnecessary losses. Finally, an IPM program reduces hazards and offers protection to the environment and producer.

USE TOLERANT VARIETIES

Always use disease tolerant or resistant varieties when they are available. Even though disease symptoms are present on a tolerant variety, the detrimental effect of the disease on yield is greatly reduced. A susceptible host is necessary for a disease to develop, so the use of a resistant variety will greatly reduce the risk of that disease becoming a serious problem.

If complete resistance is not available in a variety, using one with moderate resistance will give some protection. In addition, the effect of the resistance will be carried over to the next season because there will be less inoculum produced on the stubble.

USE PATHOGEN-FREE SEED

Pathogens may be introduced into a field on seed that has seed-borne disease organisms. Poor quality or disease infested seed may also affect the stand or plant population density, particularly under cool, wet conditions. Some diseases, such as common bunt or loose smut, are easily controlled with fungicide treatment of the seed.

In addition, poor quality seed presents a disease concern because it will not germinate and emerge as quickly as would high quality seed. This makes it more susceptible to soil-borne diseases.

ELIMINATE VOLUNTEERS

Volunteer plants can harbour diseases. Insects that carry virus diseases can move from volunteer plants to a newly emerging crop. For example, wheat streak mosaic virus is spread from wheat crop to wheat crop by the wheat curl mite. Barley also is susceptible to this virus.

While tillage systems have no direct effect on virus infection, the presence of volunteer wheat or grasses in a no-till system does increase the risk of infection. These volunteer plants are reservoirs for both the virus and the mite. The mites can only exist and live on green plant tissue. The volunteers serve as "green bridges" and allow the mite to move from these plants into winter cereals in the fall, or into spring cereals in the spring. In the process of feeding, the mite transfers the virus.

To reduce the risk of wheat streak mosaic virus in a no-till system, use the following steps:

• Control volunteer cereals and grasses with herbicide within the field to be planted and in adjacent areas, at least two weeks prior to planting.
• Avoid planting winter wheat early in the fall adjacent to spring wheat crops
• Plant spring grains early. The wheat curl mite is most active in warm temperatures. Virus infections that establish early in a cropÌs development are most damaging.
• If available, use tolerant cultivars.

PLANT AT BEST TIMES AND BEST SEEDING RATE

Early planting of spring wheat reduces the risk to foliar diseases that build up during the growing season and reduces the risk of exposure to heat stress at the 4-5 leaf stage, which adversely affects yield.

High spring wheat plant populations have high yield potential. This can produce dense canopies that increase the humidity which favours the development of foliar diseases, particularly in higher rainfall areas. To compensate, wider row spacings may be which creates a less humid environment with greater air circulation between the rows. Also, wider rows may be more desirable for handling larger quantities of the crop residues occurring with zero tillage.

In drier areas, if plant populations are too high in relation to the amount of water available to sustain good growth rates, the plants can become stressed and more susceptible to some root rot diseases. However, with zero tillage in semi_arid areas, common root rot and take-all diseases of wheat have been reduced with higher seeding rates, wider row spacings and adequate seed placed phosphorus.

BALANCE FERTILIZERS

In high rainfall areas, excess nitrogen fertilizer can produce lush growth with dense canopies. That may increase the canopy humidity, again favouring foliar diseases. In drier areas, increased levels of foliar diseases on spring wheat have been associated with inadequate levels of nitrogen. An unstressed plant is more resistant to disease than a plant stressed by low fertility or other factors. In a long-term tillage study, the severity of leaf spot diseases on wheat was, at times, higher in zero till than conventional till at low nitrogen levels. At adequate N levels, the severity was similar.12

Chloride, applied as potassium chloride, has reduced foliar and root-rot diseases of small grains when chloride levels were low (less than 30 lb/ac).13 These results vary and depend on cultivar, soil types, location, and/or the year.

MONITOR YOUR CROP

Regularly check or monitor your fields for the presence of disease so you can make appropriate decisions for fungicide use. If foliar diseases are present and yield potential is high, applying a fungicide might be useful.

Each year wheat producers struggle with the decision on whether or not to use foliar fungicides to protect their crops against tan spot and septoria leaf diseases. Reference materials for plant diseases and decision aids for foliar fungicides have been developed by various organizations.14-17

The decision guide (Table 3) here was prepared by plant pathologists in the northern plains.18 This can be used at three crop growth stages. As with all aids, this one is only a tool to help make the decision easier. The aid has no hard and fast rule, and other factors not listed, such as development of head scab, may affect the decision.

FUTURE CONTROL METHODS

Biodiversity of soil microorganisms increases with zero tillage. There are more fungi, actinomycetes, bacteria, and denitrifying microorganisms. The diversity of soil life matches the diversity of the crop rotation.

Changes in the microflora and greater microbial activity may lead to future disease control methods. Microorganisms that can compete with, or antagonize, plant pathogens may be found in the soil or residue.

Biological control methods may give protection from plant pathogens in the future. With these methods, a natural antagonist of a pathogen is applied to the crop. In winter wheat, microbial populations in the rhizosphere can increase by 50% with zero tillage and 95% of these may antagonize the fungus causing take-all.19 Mixtures of bacteria that suppress take-all have increased crop yield when applied as "living seed treatments".20 Their effectiveness varies with location and further research is needed before being used commercially.

Fusarium head blight (scab)

Scab is caused by several fungi that overwinter and persist in residue of small grains, corn, and other grass hosts. The fungus invades the crop if prolonged dew periods and high humidities occur at the time of flowering. Head blight infection can be severe with losses in yield, test weight, seed quality, and other quality factors, and the possible presence of mycotoxins. The disease is most damaging in areas of high rainfall and low diversity cereal based rotations.

Manage with:

• Choosing good crop rotations; plant wheat andbarley crops into non-host residue; planting wheat or barley into corn resi-due is a very high risk for scab!
• Using the most tolerant cultivars available.
• Planting early (very early in the case of winter wheat) and staggering maturity dates of cultivars, so that all of the crop isnÌt flowering at the same time.
• Considering the use of fungicides at flowering

"Disease management takes a whole new set of skills and a whole new encyclopedia of knowledge, especially for root diseases. Diverse crop rotations can not only help manage diseases on the surface but also in the root zone."

Daryl Domitruk,

Manitoba Agriculture, Carman, Manitoba

What is take all root rot?

Take-all is a fungus root rot which causes a dark, shiny black discoloration of roots, crowns and lower stems of wheat and barley plants. This, in turn, leads to premature whitening and death of the plant. The bleached white heads of affected plants contain little or no grain. Symptomatic plants may appear scattered, in patches, or across large areas of the field.

The fungus, called Gaeum-annomyces graminis survives between susceptible crops in undecayed crop residue, on grassy weeds, and on volunteer wheat. The disease is favoured by very moist soils, and is most commonly seen in irrigated wheat, but has been observed in dryland wheat production under moist soil conditions particularly in Australia. Take-all of wheat is more severe in winter wheat than spring wheats, with Canada Prairie Spring cultivars being more susceptible than hard red spring or durum types.

Studies on the effect of zero till production on take-all have shown varied results. Studies in northern plains states of the United States show reduced tillage produce cooler and wetter soils, which favour the fungus. In semi-arid environments take-all of winter wheat can decrease with zero tillage in contrast to moister coastal areas where the disease becomes more severe with this practice. This shows the importance of understanding a disease and its environment.

Manage with:

• Crop rotation of three years or more between susceptible crops (wheat, barley, rye, some grasses). This can be accomplished with 3 crop-type rotations.
• Maintaining balanced fertility. Ammonium forms of nitrogen fertilizer suppress take-all compared to nitrate forms. Take-all is more severe under high alkaline conditions.
• Control volunteer wheat and grassy weeds, which may harbour the fungus.
• Use more tolerant varieties.
• Some fungicide seed treatments suppress take-all.

"The best disease control method available is a healthy plant. Optimal fertilizer levels, seed placement, rotation and soil moisture will make a healthy plant which can resist diseases."
Lyle Samson,

MB-ND Zero Tillage Farmers Association, Minot, North Dakaota

Prepared from information provided by:

J.M. Krupinsky, USDA-ARS,

Northern Great Plains Research Lab

Mandan, ND 58554-0459

Telephone (701) 667-3011 Fax (701) 667-3054

Email "krupinsj@mandan.ars.usda.gov"

M. McMullen

Dept. of Plant Pathology

North Dakota State University, Fargo, ND 58105

K.L. Bailey, L.J. Duczek, and B.D. Gossen

Agriculture and Agri-Food Canada,

Saskatoon Research Centre, Saskatoon, SK S7N OX2

References:

1. Summerell BA and Burgess (1988). Austral. Plant Path. 17:88
2. Bailey KL and Duczek (1992). Can. J. Plant Path. 18:159
3. Turkington TK and Clayton (1996). Proc. Pulse Crops Wrkshp
4. Bailey KL Lafond and Derksen (unpublished)
5. Sutton JC and Vyn (1990). Can. J. of Plant Pathol. 12:358
6. Davies JM (1991). Aspects of Appl. Biol. 27:351
7. Bailey KL et al (1992). Can. J. Plant Sci. 72:583
8. Krupinsky J (1994). In: Advances in Tan Spot Research
9. Pedersen EA and Hughes (1992). Can. J. of Plant Path. 14:152
10. Ledingham RJ (1961). Can. J. Plant Sci. 41:479
11. Petrie GA (1994). Can. Plant Dis. Surv. 74:141
12. Krupinsky J (1995 & 97). Proc. of MB-ND Zero Till Farmers Wrkshp
13. Tinline RD et al (1993). Can. J. Plant Pathol. 15:65
14. A Decision Guide for Foliar Disease Control in Irrigated Wheat, Sask. Irrig. Dev. Centre, Outlook, SK
15. Berglund DR (1994). Sunflower Production Exten. Bull. 25, NDSU, Fargo, ND 58105
16. Martens JW et al (1984). Diseases of Field Crops in Canada Can. Phytopath. Soc, Harrow, ON
17. Wiese MV (1987). Compendium of Wheat Diseases Am. Phytopath. Soc., St. Paul, MN. Compendia also available for alfalfa, barley, bean, beets, peas, sorghum, and soybeans.
18. McMullen M (1996). North Dakota model adapted from R Jones, Minnesota and R Bowden, Kansas
19. Herman M (1985). Soil Tillage Res. 5:371

Pierson EA and Weller (1994). Phytopath 84: 940