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17/07/2007
Wet weather increases disease risk to veg crops
Wet weather experienced by many across the country has increased the risk of disease in vegetable crops as well as reduced the opportunity to get on the land to spray. With this in mind, growers are being advised to look very carefully at their crops and be aware of the disease risk to them, prior to planning their next fungicide strategy.
Standard guidance is to apply fungicide sprays to vegetable crops prior to diseases infecting them. However, because of unseasonably wet weather, this hasn't always been possible this season, says UAP's Chris Wallwork and, even if it has, many crops have been in a latent phase of disease infection, making it very difficult for growers to identify symptoms. He comments: "Many vegetable crops have no appearance of symptoms following initial infection but, after time, diseases such as alternaria, brassica ring spot and white blister appear. This is less true of a disease like powdery mildew in brassicas and carrots although it can still be difficult to spot its early form."
Mr Wallwork says that many growers rely on strobilurin fungicides that "do a good job" on a wide range of diseases, but the downside is that they have very little eradicant activity and are purely protectants. He adds: "When crops went in the ground after the rains then the strobs will be OK. There will be situations, however, where crops went in prior to the wet weather and are now infected with disease, but it may not be showing. In this case growers need to consider what diseases might already be in the crop and whether or not an eradicant fungicide should be used in conjunction with a strob."
To gain both eradicant and protectant activity, Mr Wallwork recommends a Signum (pyraclostrobin + boscalid) + tebuconazole tank mix or Nativo (trifloxystrobin + tebuconazole) on its own. He also suggests that a metalaxyl-based product could also be used with a strobilurin to improve the spectrum of activity.
"We've seen very clearly in past vegetable crop UAP trials in cabbages that the first fungicide application can be made on an apparently clean crop yet, after a few days, high levels of brassica ring spot have appeared," says Mr Wallwork. He says that plots treated with a strobilurin and tebuconazole mix had a very good knock-down effect and remained almost clean, while those with just a strobilurin on their own looked good at the heart and on central leaves but the outer leaves were very spotted. Triazoles alone gave a clean crop but not as smart as the strobilurins. "The combination plots were the best of the lot," he adds.
Mr Wallwork says that in a season like this it is very difficult to plan and this has been reflected in UAP with colleagues in the east of the country experiencing conditions more in tune with what might be expected in the west and south west of the country. "Growers can't assume they will get the same results as last year and they must not plan their strategy by a set blueprint. Many diseases in vegetable crops will be in their latent phase now – so check what diseases are already present, think about what diseases were in the previous crop or in the weeds and plan carefully before going out to spray," he concludes.
 | Chris Wallwork |
Standard guidance is to apply fungicide sprays to vegetable crops prior to diseases infecting them. However, because of unseasonably wet weather, this hasn't always been possible this season, says UAP's Chris Wallwork and, even if it has, many crops have been in a latent phase of disease infection, making it very difficult for growers to identify symptoms. He comments: "Many vegetable crops have no appearance of symptoms following initial infection but, after time, diseases such as alternaria, brassica ring spot and white blister appear. This is less true of a disease like powdery mildew in brassicas and carrots although it can still be difficult to spot its early form."
Mr Wallwork says that many growers rely on strobilurin fungicides that "do a good job" on a wide range of diseases, but the downside is that they have very little eradicant activity and are purely protectants. He adds: "When crops went in the ground after the rains then the strobs will be OK. There will be situations, however, where crops went in prior to the wet weather and are now infected with disease, but it may not be showing. In this case growers need to consider what diseases might already be in the crop and whether or not an eradicant fungicide should be used in conjunction with a strob."
To gain both eradicant and protectant activity, Mr Wallwork recommends a Signum (pyraclostrobin + boscalid) + tebuconazole tank mix or Nativo (trifloxystrobin + tebuconazole) on its own. He also suggests that a metalaxyl-based product could also be used with a strobilurin to improve the spectrum of activity.
"We've seen very clearly in past vegetable crop UAP trials in cabbages that the first fungicide application can be made on an apparently clean crop yet, after a few days, high levels of brassica ring spot have appeared," says Mr Wallwork. He says that plots treated with a strobilurin and tebuconazole mix had a very good knock-down effect and remained almost clean, while those with just a strobilurin on their own looked good at the heart and on central leaves but the outer leaves were very spotted. Triazoles alone gave a clean crop but not as smart as the strobilurins. "The combination plots were the best of the lot," he adds.
Mr Wallwork says that in a season like this it is very difficult to plan and this has been reflected in UAP with colleagues in the east of the country experiencing conditions more in tune with what might be expected in the west and south west of the country. "Growers can't assume they will get the same results as last year and they must not plan their strategy by a set blueprint. Many diseases in vegetable crops will be in their latent phase now – so check what diseases are already present, think about what diseases were in the previous crop or in the weeds and plan carefully before going out to spray," he concludes.
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