Testing for spray strength

7 March 1997

Testing for spray strength

By Andrew Blake

A SIMPLE test to tell when sprayed fungicides are no longer working may offer growers better disease control and help trim costs.

Currently being researched by the SAC with £250,000 of HGCA levy cash, the three-year project mimics diagnostic techniques used in eyespot and septoria detection kits.

The test is based on antibodies under development at the Central Science Laboratory, says SAC pathologist Dr Simon Oxley. "It was the outcome of some fairly simplistic thinking. If we can detect eyespot, why not fungicides?" Diagnosing hidden diseases by the ELISA method has always been tricky because they tend to occur sporadically, says Dr Oxley. By contrast growers know precisely when they have applied fungicides – the big uncertainty is knowing when a particular treatment has run out of steam.

Useful tool

"If we can use diagnostics to detect the amount of fungicide remaining and relate that to levels of disease eradication, we could have an extremely useful tool."

Using it could allow growers to time following treatments much more accurately without risking loss of disease control, he explains.

Triazoles have been chosen as the first target, not least because of their widespread use and their systemic broad spectrum properties. Their application in mixtures, both with morpholines for cereal mildew and, as expected, with the newer strobilurins, makes them useful as potential "markers", he adds.

First attempts to find suitable antibodies were unsuccessful. "But CSL staff know why that was and have already produced an antibody for tebuconazole." The hope is that others will follow, although a single generic test for a wide range of older triazoles may not be so easy, admits Dr Oxley.

As with all ELISA tests, a simple colour reaction gives the researchers an idea of the amount of fungicide remaining after a given application.

Key thrust of the current work, still in its first season, is to relate detected fungicide levels to amounts of disease present. "We have freezers full of leaf samples sprayed at various stages to help provide the answers now that the antibodies are coming on stream."

Riband wheat is being used, and the exploratory diseases are currently confined to brown rust and Septoria tritici. "They have different sensitivities to the triazole," explains Dr Oxley.

"We aim to use glasshouse work to find out when fungicides are no longer effective and the plant is relying 100% on the inherent resistance of the variety."

By using a range of dose rates and diseases inoculated at various times the researchers can examine new growth. From there it should be possible to see how much fungicide is getting into the leaves and relate that back to a given degree of protection.

Adequate control

"More importantly it should allow us to say you need x to bring you back to adequate control.

"I am optimistic that we have something useful coming through. The concept is the easy bit. Putting it into practice in the field may be trickier. Thats because we have no idea of whether the fungicide concentration in leaves will be the same in the thick parts of crops as in thinner ones."

However if the work can more accurately tell growers when to re-spray it will have proved well worthwhile, he believes.

The novel HGCA-funded idea could tell when fungicide sprays are running out of steam and make subsequent treatments more precise.

Glasshouse tests are needed to validate the fungicide diagnostics idea, says Simon Oxley. Fungicide Special continues on page 67.


&#8226 New CSL antibodies.

&#8226 SAC ELISA tests.

&#8226 Detecting triazoles.

&#8226 Link chemical/disease levels.

&#8226 Should aid re-spray timing.

&#8226 Could help cut costs.

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