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Anthracnose

Introduction

Anthracnose, Colletotrichum cereale, was a disease first discovered in the 1950's. It is potentially very damaging and, once identified, efforts should be made to control the outbreak straight away.

The disease can be present in one of two forms. Both forms often become evident after an initial yellowing of individual grass plants. These lesions first appear small but can reach up to 15 centimetres in diameter. The appearance of the lesions can be highly variable. They can appear in circular patches, like fusarium patch, or in small irregular patches.

One form of the disease is known as foliar blight. This is where the fungus affects the leaves of the grass plant. Often, dark acervuli structures produced by the pathogen can be seen on the leaf structures. This can lead to eventual death through a greatly reduced photosynthetic capacity.

Basal rot is the other form of the disease. It is probably fair to say this stage of the disease is more widely known, as many refer to anthracnose simply as 'basal rot'. However, as you can see, this is an inaccurate description. At any rate, infected plants can be easily pulled from the sward by hand as the leaf sheath, crown and any horizontal growth exhibit signs of fungal structures and, eventually, necrosis. It is important to remember that as soon as basal rot becomes prevalent, there is less chance of effective control. As we shall see later, a preventative approach is more important than a curative one.

Causes of disease development

An important point about anthracnose is that although it can potentially affect different cultivars, Poa annua and Agrostis stolonifera* are most susceptible to the disease. This may partly explain the increasing significance of the disease on British soil over the past 5/10 years.

Another factor that may explain its increasing prominence is that it is often caused by large amounts of physiological stress. Physiological stress manifests itself in many ways for a grass plant, and has many causes. Of these, large volumes of traffic, poor nutrient and water availability, close mowing and excessive thatch are amongst the major culprits. This is why, for repeat-offending playing surfaces, the pathogen is often present in the same areas. Popular walk off points on putting greens and south-facing embankments are renowned for exhibiting signs of stress, and therefore this particular disease.

It is known that there are different climatic triggers for the foliar and basal forms of the disease. Both prefer humid conditions, but the basal rot form often becomes present during periods of cool and cloudy weather, whereas the foliar blight form is most active during hot summer conditions. The method of spread is via water splash or any cultural practice that induces physical disruption to the turfgrass sward, such as mowing, rolling etc.

The life cycle of the fungus is as follows:

  • Colletotrichum cereale overwinters as mycelium or conidia in thatch layers (It may also survive as aggregates of stomata that are formed on stolons) (A)
  • Exposure to sunlight and warm temperatures (>15°C) causes production of conidia (Spring/summer) (B)
  • Conidia are spread by water or physical interaction (C)
  • In warm, wet condtions in early to mid-summer, the conidia may also germinate on plant surfaces that develop into melanised appressoria.
  • These appressoria anchor the fungal filament to the plant surface.
  • The appressoria then pierces plant epidermal cells with a 'penetration peg', and produces single hypha.
  • These hypha then invade surrounding plant mesophyll cells, killing them with enzymes and/or toxins before mycelia growth. At this point yellow foliar lesions and black acervuli develop. (D)

(Description found here)

*Most modern varieties of Agrostis stolonifera show excellent resistance to anthracnose

Chemical Methods Of Control

Chemical control of anthracnose will in most cases be a curative action. At present, there are a number of products one can choose from for effective control. Examples of the products available that provide excellent preventative and curative action include Instrata, Chipco Green or Heritage Maxx. Applying these in conjunction with a dose of phosphite has been shown to tackle the disease very well. It is important to remember the following when combating anthracnose as a disease:

  • Where possible, change active ingredients every few applications to reduce the chance of pathogen resistance. Just as the over-use of antibiotics in treating human conditions has lead to increased bacterial resistance, this can occur in fungal pathogens of plants with increased exposure to the same chemicals.
  • Apply in calm, dry conditions with a medium-fine spray. As the disease is mostly found on the plant surface, spraying coarse droplets will not allow the chemical to stick to the plant as wanted for effective control.
  • Follow the recommended application rates at all times. Do not exceed or apply at lower rates than the recommended dose on the product label.

There are other chemical preventative measures that can be taken. For example, using products such as Oxy-rush and Pervade. Oxy-rush effectively helps turn anaerobic conditions into aerobic conditions and thereby increases the amount of oxygen for the indigenous beneficial micro-organisms. This then aids increased rates of thatch and black layer reduction and provides aeration of the upper horizons of the soil through the production of beneficial gases rather than hydrogen sulphide. Pervade will help reduce stress by pulling water through the profile, moving it away from the surface and rooting areas during periods of heavy rainfall. This reduces moisture stress by facilitating increased infiltration rates. Both products can therefore help reduce the incidence of anthracnose by reducing some of the stresses on the sward.

Traditional wetting agents in summer months can often hold moisture too high within the soil profile, suffocating plants and slowing infiltration rates during periods of intense rainfall. During periods of summer stress look to wetting agents, such as Qualibra, that hold water at depth. There is no benefit from holding the water within the top horizons of the profile, as this essentially suffocates beneficial bacteria.

Cultural Methods Of Control

The most important thing is to try and reduce the physical stress the individual plants are under. The first step is to ensure you personally understand what is going on below the surface. Taking a soil sample and understanding the nutrient balance of your soil is an excellent starting point. Pitchcare technical advisers can provide a soil sampling service where required. From here, a fertiliser programme can be written that will suit the soil texture and imbalances that your sward is experiencing.

The growing medium/rootzone needs to be free-draining and not compacted. The aim should be to effectively shatter the soil structure in vertical and horizontal directions, thereby increasing infiltration rates, and allowing easier growth of root systems through the profile. How often this occurs and which type of machinery to use will be dictated by your own personal situation.

Thatch reduction should be a key to any successful sward growth and so deep scarification/verticutting programmes should already be in place. Particularly so in swards comprising mainly rhizome or stolon forming species, as the thatch build up is quicker than for swards comprising other species (e.g. Lolium perenne). Keeping thatch levels below 8% will allow easier water infiltration and reduce disease pressure. Again, the frequency and depth of thatch removal should be allied to your own personal situation. Remember, thatch removal should not occur when the plant is already under stress, such as drought or heat stress, as this could only worsen the problems.

During the winter try to ensure mowing/rolling frequencies are reduced as much as possible, and where possible hand mowers are used. These can have significantly less weight than many triple mowers in the same circumstance. Rolling should not occur unless absolutely necessary outside of the regular growing season, when there will be significant benefit for play. Once the soil starts to hold more water, a compressing action can have a marked affect on the compaction of the soil. Change directions of cut/rolling where possible.

It is also important to ensure good air-flow, ensuring removal of surface moisture by environmental factors.

Summary

In summary, Colletotrichum cereale, is increasingly becoming one of the most important diseases in sports turf. Reducing the amount of stress the grass plants are under will provide the key to reducing the severity of disease outbreaks. Whilst there are products with curative actions available, where possible try to prevent the cause of the disease by providing the best possible environment within which the grass can grow.


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