New technologies for the early detection of pests and diseases were discussed at a recent Association of Applied Biologists meeting in Lincolnshire. Louise Impey reports.

Air sampling is proving to be a useful method for identifying and quantifying the presence of pathogen inoculum, and is increasingly being used to guide spray decisions at the farm level, said Jon West, of Rothamsted Research.

“Automated detection of what’s in the air gives you advanced warning of airborne spores, which is how most diseases are initiated,” he explained. “Even for common diseases, the timing of disease onset can vary each year and is often the difference between good and poor control.”

When air sampling is integrated with appropriate diagnostic methods, species that could not be identified by visual methods before are now being picked up, he added.

“A good example of this is the new diagnostic for sclerotinia in oilseed rape,” he reported. “In 2007, there were elevated levels of pathogen DNA in the air, which was unexpected because it was unusually dry.

“This coincided with the highest incidence of sclerotinia stem rot for more than 10 years. The same occurred in 2011, which was another very dry spring. It has shown that spores can still be found in the air in those conditions, contradicting previous knowledge.”

However, a reliable warning service also needs to be able to deliver a rapid answer on-site, so that spray decisions can be made in time, he added. “We still need technology which can do that. Understanding the weather conditions and how they might affect the dispersal of spores is another part of the picture.”

Another development, electronic nose technology, has the potential to detect the volatile signals produced by plants in response to attack, revealed Nigel Paul, of the University of Lancaster.

“All plants emit a mixture of volatile compounds,” he said. “They are characteristic of a particular species and they change in response to environmental factors, including the arrival of pests and disease.”

Predators that attack crop pests use the changes in volatile signals to detect their prey, he explained. “This suggests there’s a way for us to be able to detect attack and treat the crop accordingly.”

Complex and expensive equipment such as gas chromatogram mass spectrometry can be used, he said. “But we are more interested in e-noses, as they are cheaper and easier to use and can be linked to automatic interpretation software.”

An HDC-funded project carried out by Prof Paul confirmed the technology could differentiate between crops and their different volatile signatures.

“They are capable of the necessary discrimination and can act as diagnostic tools for pest and disease attack,” he concluded.

However, as the work was done in a laboratory, in a clean and confined space, there are still many challenges to overcome. “Scaling it up and applying it to the crop environment, rather than just to the plant, remains to be done.”

Optical disease detection

Remote disease detection can be done by a variety of optical techniques, including NIR reflectance, imaging methods and chlorophyll fluorescence measurement, said Jon West.

“If disease is already established in the crop, foci can be picked up and selective spraying done to prevent it spreading any further,” he said.

NIR reflectance is the most cost-effective, he stressed, and can measure the health of the crop canopy, picking up any leaf symptoms. “It can also detect if there is less leaf tissue present due to disease. The ideal would be for it to be able to give us a disease index.”

Imaging methods allow users to be able to discriminate between diseases and other stress factors, he said.

“Thermal imaging, for example, can be used to detect the early stages of infection, as they tend to produce ‘hotspots’.”

However, unless the disease can be picked up at this stage, the whole field has to be sprayed anyway, he admitted. “It may be that this technology is more relevant to the greenhouse than the field.”