Diseases and pests can be detected in greenhouse atmosphere

The alarm substances that plants emit when they are attacked by pests or diseases can be measured in the greenhouse atmosphere. This effect has been used to design a system to promptly detect greenhouse pest and disease outbreaks. This development was reported by Wageningen farm technologists and plant physiologists in the Annals of Applied Biology.

The researchers based their work on a discovery made by Wageningen entomologists: plants that are being eaten by insects emit a signal substance with which they attract the natural enemy of that insect. In the new study, the side shoots of tomato plants in a greenhouse were removed. The volatile alarm substances that were released were captured and analysed, explains the lead author Roel Jansen.

The researchers discovered three groups of volatile alarm substances. First, there is a group of alcohols that are released when the cell membrane of the plant is damaged, and secondly a group of terpenes – oily substances – that are released when the leaf hairs (trichomes) of the plant are damaged. 'These are interesting substances,' says Jansen, 'but they are also released when you touch or harvest fruit from a plant. This is confusing, because you want to know specifically when a plant is being affected by a pest or disease.' 

Luckily, there is a third group of hormone-like substances, including methyl salicylate, which are released if plants are attacked by pathogens or insects. The concentration of these substances in the atmosphere increases if plants are attacked by pests or diseases, but not when fruit is harvested. 'These are true stress hormones', explains Jansen.

'We now know which substance are released when plants are attacked by insects and diseases, and in which concentration', he continues. As a result, the basis has now been established for a sensor that can detect infestations of pests or diseases in the greenhouse. But additional research is necessary before a manufacturer can actually build this detector. 'We are first going to conduct up-scaling calculations', says Jansen. 'We now know the concentrations of alarm substances in a small greenhouse of 40 m². But what will happen if the greenhouse is 10 or 100 times as big? We can estimate this with models, but such models contain assumptions. Therefore, we want to conduct a new series of measurements in a properly controlled greenhouse at a commercial scale, for example at Wageningen UR Greenhouse Horticulture in Bleiswijk.'

Jansen foresees a demand for this detection system. 'The guidelines for using crop protection agents are becoming increasingly strict. If you promptly detect the presence of pests or diseases, you need smaller amounts of agents to control the infestation. The trend in horticulture is towards fewer greenhouses, but much larger ones. Due to the larger scale, the outbreak and spread of pests or diseases is becoming a bigger risk for the growers, while at the same time they are often unable to monitor all the plants in the greenhouse. As a result, there is a need for an automatic alarm system.'

Jansen has been studying the detection of alarm substances from greenhouse plants for four years. 'We began in the laboratory, with damaged bits of plants in small dishes.' He will soon defend his dissertation on this research at the Farm Technology Group. His research took place in close cooperation with the Plant Physiology and Organic Chemistry groups in Wageningen and the German research centre in Jülich, which has a facility for measuring alarm substances under extremely controlled conditions. / Albert Sikkema