T
he definition of plant disease includes anything that adversely affects plant health. This definition can include such factors as nutrient deficiencies, mower damage, air pollution and pathogens, including nematodes and soil pests. Symptoms and signs are used to diagnose the condition of a plant. Symptoms are the physical characteristics of disease expressed by the plant. These can include wilt, galls, cankers, rots, necrosis, chlorosis and general decline. Other signs of disease presence can include fungal fruiting bodies (such as mushrooms), mycelia, bacterial slime, presence of nematodes or insects, or the presence of insect holes and root damage.
Once you have determined that a real
problem exists, and is caused by a living organism, you need to decide what type of organism may be causing the damage. Begin by establishing which plant part or growth stage is showing symptoms. Are symptoms showing on roots, tubers, bulbs, corms, seedling, foliage, stem, branches, trunks, flowers, fruit or on the entire plant?
Diseases can affect many parts of the plant: root symptoms include galls, discoloration or death to parts of roots; death of the entire root system or just feeder roots is indicative of many fungi diseases. Injury to the root system often includes yellowing of foliage, stunting or wilting. Seedling diseases, where the seedlings fail to emerge or die off, is referred to as damping off disease. Fungi such as Rhizoctonia, Pythium, and Fusarium are common and affect seedlings just at or below the soil line. Plants with dusty looking white growth are diagnosed as
having powdery mildew (Erysiphe spp.) or downy and damp looking - downy mildew fungi (Phytopthora spp.). Leaf symptoms are seen in many forms: discolouration (yellowing or shades of green), localised or indistinct patterns, usually indicates a virus; dead areas on leaves can be caused by fungi or bacteria. Necrotic areas caused by fungi may contain hyphae or fruiting bodies, particularly after incubation in a warm, moist environment.
Understanding the specific disease, or the life cycle of the pathogen involved, is necessary to ensure effective controls can be implemented. Three major factors contribute to the development of a plant disease: a susceptible host, a virulent pathogen, and a favourable environment. - the disease triangle. A plant disease results when these three factors occur simultaneously. If one or more of these factors do not occur, then the disease does not occur. The genetic makeup of the host plant determines its susceptibility to disease. This susceptibility or resistance may be determined by various environmental, physical and biochemical factors. Plant stature, growth habit, cuticle thickness, and stomatal shape are a few physical factors that influence disease development.
The plant’s developmental stage may also influence disease development. Pathogens differ in their ability to survive, spread, and reproduce. Environmental extremes of temperature, light, or moisture can accentuate many diseases. Cool, moist conditions are ideal for many fungal pathogens. Understanding the disease cycle is
important when considering control options. Learning the chain of events that contribute to a disease helps point out the weakest links. Control measures can then be used to break the cycle. Most pathogens must survive an adverse period, usually winter, when they do not actively cause plant diseases.
Disease Cycle:
The pathogens that cause these diseases are always around and often laying dormant in the thatch layers waiting for the ideal conditions in order to become active. Once these spores are activated, and have found an acceptable host, they are able to grow and reproduce themselves, spreading new spores and infections to other areas of turf. This cycle continues whilst the right conditions prevail. Spores are spread by wind, water, and by traffic. It is during periods of fluctuating weather, particularly changes in temperatures, that an outbreak of disease takes place. Attacks of soil borne disease can appear at any time of the year, foliar diseases prefer warm humid conditions of the spring and summer months.
The graphic below details the most common diseases found in turfgrass environments in the UK. The details are only a brief introduction to these diseases and their control. There are many good books available that can be obtained from specialist book sellers and organisations that detail and show pictures of these diseases and treatment recommendations.
Other turf grass plant diseases are Rusts, Fairy rings, damping off disease,
Anthracnose Colletotrichum graminicola Symptoms and conditions
Seen between late summer, late winter, appears as yellowing individual annual meadow plants in the sward. The disease causes distinct rotting at the base of the plant. The disease is prevalent on compacted/wet soils.
Control
Keep the soil well aerated and free draining and implement a IPM strategy.
Fungicide - carbendazim, iprodione, chlorothalonil
Brown Patch Rhizoctonia solani Symptoms and conditions
A soil borne fungus usually present in most soils. The disease is seen as water soaked patches. The disease is usually confined to leaf tissue but, on severe attacks, will affect the whole plant.
Control
Brown patch can be prevented by regular scarification which removes infected thatch layers. Regular brushing to remove leaf wetness also helps keep the disease at bay. Implement a good IPM strategy.
Fungicide - iprodione
Dollar Spot Scelerotina homoeocarpa Symptoms and conditions
On close mown turf seen as very small spots 10-20mm diameter and can coalesce into larger areas, with fescue grasses being most susceptible hosts, often resulting from having low fertile soils.
Control
Dollar spot can be reduced by selecting resistant grass cultivars, maintaining healthy swards and keeping a good nutrient status in the soil, coupled with a IPM strategy.
Fungicide - carbendazim, chlorothalonil, iprodione, azoxystrobin
Fusarium Patch Microdochium Patch Pink Snow Mold Microdochium nivale Symptoms and conditions
Damaging and disfiguring disease that occurs on amenity turf. 50mm orange/brown spots that can rapidly increase in size.
Control
Control moisture to avoid humid conditions, keep surfaces brushed and aerated, and carry out a IPM strategy.
Fungicide - azoxystrobin, mycobutanil, iprodione, chlorothalonil, tebuconazole + prochloraz, propiconazole, carbendazim
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