Monday, 3 July 2017

Looking closer at fusarium disease outbreaks

I'd like to communicate some information about disease outbreaks and disease control. It is quite topical because the past week has, due to rainfall and humidity, prompted some fusarium patch (michrodochium nivale) disease on the greens.

It is important to understand the reasons for an outbreak of disease and a simple but effective diagram is the disease triangle, below:

Basically, as the diagram demonstrates, disease occurs because a pathogen (fusarium), environmental factors (moisture, humidity, thatch layer etc) and a susceptible host (poa & bent grasses) are present at any particular time. If any of the factors are not present, then disease will not occur. 

For example, it is extremely unlikely that fusarium will occur during hot, dry weather because the disease favours humid, moist conditions. Given, the pathogen will be present at any time and the host (grasses) are always present but without the correct environmental conditions, an outbreak will not occur.

Fusarium can occur at any time of the year, providing it has favourable conditions to do so.

Control of disease can be addressed either culturally or chemically.

Cultural control includes dew removal (mowing, switching, brushing), well timed aeration (grooming, verticutting, scarifying) to encourage a less humid grass surface, regular aeration and topdressing to provide a drier surface, overseeing with grass cultivars that are more disease resistant, thatch removal (scarifying, hollow coring) as disease pathogens inhabit the thatch layer, irrigating only when necessary and fertilising correctly. 

We are aiming, always, to keep putting surfaces DRY. Fusarium requires MOISTURE. Hence why so much of the work over the last 3 years has been targeted towards providing greens that are firmer, less thatchy and drier. Essentially, cultural control forms part of an annual greens maintenance program.

Chemical control is essentially applying a fungicide to achieve control. It is generally a last resort because it is expensive. The other issue is that, legally, fungicides can only be applied so many times per year and this will generally be determined by government and industry bodies. The product label will specify the maximum number of doses that can be applied in any year. Beyond that, an alternative fungicide (generally with a different active ingredient) will have to be used.

So it becomes evident that common use of fungicides can become expensive. That is why we first aim to achieve control with sound turf management practices.

Secondly, we look at the time of year that the disease occurs. Although an outbreak can occur at any time, summer and autumn are not as problematic as winter and spring. This is because of growth. Fusarium is most problematic during winter because there is often very little growth and recovery from grass due to low temperatures. 

Conversely in summer, disease may be present but regular growth will help with disease control because regular mowing will generally remove the affected grass leaves. Importantly, an application of fungicide in the summer will often be wasteful because it too can quickly be removed out of the grass plant by mowing. That is why we are less concerned about disease occurring in the summer as well as the obvious point that applying a fungicide that will give control for 10-14 days in the summer is a complete waste of money. Far better to deploy fungicides during winter time when the grass needs assistance due to lack of growth and recovery.

As a point of interest, I've included a diagram of the fusarium disease life cycle below. This explains more about using different types of fungicides at different stages of the disease.

A preventative fungicide approach can be useful, particularly in autumn and winter because it will help to control disease before an outbreak occurs. 

A curative fungicide approach targets disease when it has progressed (stages 2 & 3).

The use of a fungicide with eradicant mode of action is deployed at stages 4 & 5 when the disease is advanced and causing turf loss. 

What we always have to consider is cost because chemicals are extremely expensive to purchase. With our budget, we cannot afford to be continually spraying fungicides to control fusarium. There is simply not enough money within the budget to do so.

Secondly, and maybe this is more a moral viewpoint, is that we have to ask ourselves if continuous fungicide spraying is environmentally friendly. To which the only answer is no. Additionally, we have to consider pathogen resistance to repeated use of the same fungicides.

Just as penicillin has been over prescribed by medical practitioners over the years, causing bacteria to become resistant to it, disease pathogens in turf can develop resistance to over use of fungicides because like everything else that lives, they adapt.

So, in conclusion, we do not enjoy to see fusarium disease. But as we have a limited budget, we must utilise fungicides sparingly and at the most critical times during the year. If we can achieve control without resorting to chemicals then that is the best outcome financially and environmentally.

It is a bit like type 2 diabetes which is linked to excess sugar in the diet and insulin resistance alongside other elements of a poor lifestyle. If we stay healthy, exercise regularly, limit sugar consumption then we are less likely to need the last resort (insulin injections). Prevention is better than a cure, as the saying goes. Or, perhaps cultural control is better than chemical control.