December-month for disease in 2004

Turf Diseases

By Laurence Gale Msc

With the heavy morning dews and the more recent wet weather coupled with the slow down in grass growth have been sure ingredients for fungal attacks. The right conditions to trigger these disease attacks are a weakened and susceptible plant, a disease-producing organism (pathogen usually fungi) and a fungi welcoming environment (moist, mild conditions).

The majority of diseases that are occurring now have responded to the unusually warm, winter weather conditions. Boundary layers around the leaves have stayed very moist and humid. Relative humidity is important for spore germination and penetration of leaf tissues and constant wet conditions will allow the development and transportation of active fungi spores.

Most fungi grow well between 10°C - 40°C and function best at a pH range of 4-7pH. The lack of cooler weather and sharp frosts has not helped in killing off these active pathogens.

The first step in turfgrass disease management is identifying the true nature of the problem. Diseases are only one cause of turf loss, and disease control measures will do nothing to alleviate damage from other causes such as management, wear or plant stress. It is therefore essential to determine whether the problem is disease, and if so, which disease.

The three disease factors (susceptible grass / host, pathogen, and environment) provide the sources of information for disease diagnosis. Symptoms are the expression of the susceptible grass to the disease and can take on a variety of forms.

Symptoms may appear on the leaves as small, circular, tan-coloured lesions surrounded by brown or purple borders (leaf spotting); as yellow, red, or tan blotches over most or all of the leaf blade (blighting); stunting; wilting; or as a brown or black rot on the crowns and roots. The appearance of these symptoms will also vary depending on the severity of the attack and the developing stage of the disease.

Early recognition of the symptoms is essential for good disease management, however the best form of management is using preventive cultural turf maintenance methods that reduce the ideal environmental factors that these diseases latch onto for development. I.E. regular brushing/switching of the grass to remove excess moisture, regular aeration to allow gaseous exchange and water percolation.

Removal of thatch (that helps harbour pathogens) by verti-cutting and end of season scarifying. Checking mower blade settings ensuring a sharp and precise cutting of the leaf blade.

Identification of these diseases can sometimes be difficult in the early stages of attack, its often only possible so recognise the type of disease when the fruiting bodies of the disease produce structures such as spores, mushrooms, or mycelium (small, thread-like filaments produced by fungi) that can be seen without the aid of a microscope. A good example of this is Red Thread (Laetisaria fuciformis) where the distinctive red filaments can be seen amongst the grass.

Site characteristics and turf management practices are important in disease diagnosis. Air movement, drainage, soil conditions, the amount of sun or shade, slope, and nearness of other plantings or buildings coupled mechanised and feeding programmes with inadequate mowing, fertilisation and aeration work programmes all may be important in influencing the development of turf diseases.

Its important to remember that pathogenic fungi can survive and remain in a dormant state in plant debris and soil until favourable conditions arrive again to stimulate another disease outbreak.

The Pathogens that cause these diseases are always around and often laying dormant in the thatch layers waiting for the ideal conditions 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.

Understanding the disease cycle and implementing works that can break up the disease cycle will help reduce the opportunities for disease development and outbreak.

However, we must recognise that by achieving good cultural practices to reduce thatch, improve air movement in soils (aeration), correct balanced feeding and correct mowing regimes will play an important part in promoting a vigorous healthy turf that can fight off and reduce the chance of disease attack.

Good cultural practices may be the only option for disease control in the future especially as there are moves to reduce the amount of chemicals and fungicides available for use in the coming years. However, at the moment we are still able to reduce fungi pathogen populations by applying fungicides, which either kill off the pathogen or slow down the production of fungi spores.

Again, it is important to have identified the disease correctly, so that an appropriate fungicide can be selected. Using the wrong fungicide or wrong application rates will and can lead to a number of problems, not only would it be a waste of time and money, the effect on the disease is likely to be negative and may well exacerbate the problem by making the disease more resilient to the chemical applied.

Fungicides can be divided into two broad categories: contact fungicides and systemic fungicides. The contact fungicides generally are applied to the leaf and stem surfaces of grass plants. These materials may or can be washed or mowed off easily, which implies that they often only have a short term active durations, between 7-10 days. These fungicides are usually used to control foliar diseases and not diseases of the root and crown structures of the plant. Contact fungicides are used throughout the twelve months of the year.

Systemic fungicides work in a different way the chemicals are absorbed and translocated within the plants tissues. Thus, they are not as likely to be removed from the plant by rainfall and mowing. Therefore they are active for longer periods and can protect plants for up to 4 weeks. Most systemic fungicides can control both foliar and root / crown diseases. Use of systemic fungicides during colder months is not advised, as the plant is either dormant of slow growing and will not rapidly uptake the fungicide-greatly reducing it's effectiveness.