19 September 1998

SPROUTS WORST ENEMIES

Cutting blemish diseases from Brussels sprouts with correctly targeted sprays could boost grower returns. Lucy de la Pasture reports on an aid to timing.

THREE major diseases of Brussels sprouts – alternaria, ringspot and white blister – have one thing in common, the blemishing which can make buttons unmarketable.

Improving the timing of fungicide applications is the key to reducing disease in the crop and disease development forecasting can help achieve this, believes Dr Roy Kennedy, senior researcher at Horticulture Research International (HRI) Wellesbourne.

Using weather data collected in Brussels sprout crops to predict disease development is still in its infancy, but HRI now has models for the two major blemish diseases – alternaria (dark leaf spot) and ringspot – which are currently being evaluated in commercial sprout crops.

"Results from last years alternaria trials were very encouraging. On most sites the model at least equalled the growers disease control with less fungicide used," says Dr Kennedy.

Both models work in two parts, the first predicting when conditions for disease infection are favourable. The model output appears as a bar chart which indicates the risk of infection each day and can be used to time crop inspections to coincide with the first appearance of disease in the crop.

"The older leaves in the crop should be inspected as disease can develop more easily on mature tissue and symptoms will appear first on these," says Dr Kennedy.

The second part of the model is triggered when disease is first seen, or is thought likely to be seen, in the crop. It calculates optimum spray timing for control. Disease development is predicted using the data collected from within the crop – leaf wetness, temperature, relative humidity and rainfall.

Although both thought of as wet weather diseases, alternaria and ringspot have different environmental requirements for infection.

There is some controversy over the specific factors affecting ringspot development. Dr Kennedy is at odds with the Dutch who have a criteria in their model of 3-4 days, during which there are 18 hours of each day with a relative humidity of over 90%.

"Ringspot can infect crops as long as there is a period of high humidity for at least four hours, even in the absence of wetness," explains Dr Kennedy. "Conditions for ringspot during the season are not often limiting, unless there is a very dry spell. The number of spores able to cause infection will be greater the longer the period of wetness, but even one day of rain can facilitate high levels of infection occurring."

"Showery or thundery conditions with temperatures of 15íC are very favourable for infection with ringspot," explains Dr Kennedy. "Alternaria requires much longer periods of wetness, a minimum of 10 hours in duration, and temperatures of 15-20íC."

The alternaria model bases its predictions on the epidemiology of A. brassicae which is the most important species affecting long-season sprouts. These are in the ground for a long period of time and are at greatest risk from wet weather diseases.

But dont expect to put your feet up and let the computer make the decisions. Regular monitoring is an integral part of using the model, warns Dr Kennedy.

Both diseases produce spores which require a drop in humidity to become air-borne and this will occur on most summer days.

"The models can only forecast disease development using the environmental data collected in the crop. Air-borne spores can blow into the crop and cause infection which could be at a different stage of development to that already being tracked in the crop by the model," explains Dr Kennedy.

Ringspot can be a particular problem in this respect. "We believe that air-borne spores are the only spore form produced and they can travel long distances very easily," says Dr Kennedy. "They have a covering which makes them able to resist drying out during falls in relative humidity and maintain their viability."

However, the model can deal with this imported infection provided it is observed in the crop. New infection can be marked and a spray date will be calculated to give an optimum timing to control both ringspot populations.

In the future air-borne inoculum shouldnt pose such a problem. New systems are being developed which can catch spores in the air and it is possible that a quantitative prediction of disease likely to occur in the crop could be made from this information.

"This would help to determine which would be the best product to use and what dose would then be appropriate," explains Dr Kennedy.

Once the spray timing part of the model is running, spray thresholds are reached according to the anticipated development of the disease. Thresholds have been developed and tested over a number of years in replicated trials to find optimum timings for fungicide application.

"Initially thresholds are directed towards reducing inoculum on the leaves of the crop," explains Dr Kennedy. "Further thresholds later in the season protect the impact of disease on the buttons."

Fungicide sprays are in practice often applied to the crop as part of a regular programme. The problem with being locked into a programme is that sprays can be applied either too early or too late to reduce inoculum.

"Timing sprays to coincide with sporulation will use sprays more effectively and reduce the amount applied to the crop in the season," comments Dr Kennedy. That must be good news to growers and supermarkets alike.