Sometimes it seems as if growers are fighting the same crop issues, year after year, perhaps opting for new disease-resistant cultivars to try to get ahead of the problem. But there are innovations in natural vegetable disease control that have shown promise. One of those developments is the study and potential use of volatile organic compounds (VOCs).
VOCs are naturally-occurring chemical compounds emitted from plants that serve as a means of communication. As plants emit VOCs, they send signals into the environment. VOCs are composed of carbon and emitted in gaseous form. Some VOCs – the terpenes – give flowers their scent. VOCs send signals which attract pollinators, repel pests or protect against abiotic stressors.
The organisms in a plant’s microbiome also emit their own VOCs. Some of these, in conjunction with the plant’s own VOCs, contribute to create a healthy growing environment. But often, chemical crop protectants can interfere with these VOC signals, inhibiting the plant’s own natural communication and defense strategy. When the naturally-occurring microbiota which surround a given plant are eliminated, their contribution to the plant’s communication strategy is disabled.
Without certain microbiota, a fruit or flower will smell differently. This change can cause pollinators to disperse or attract pathogenic insects or microbes, potentially increasing the disease risk and requiring even more chemicals to keep problems at bay.
So how can VOCs be used by growers to combat stressors leading to disease? VOCs are protective and are released from the plant to attract and create a healthy biome to protect against stressors. And stressors, such as the presence of certain insects, microbes, weeds or even memories of past stressful events, can trigger specific VOC release from the plant. Plants have the ability to remember past stressors, and become “primed” to respond when similar situations are again encountered. VOCs are known to play a role in this priming response.
VOCs prime plants’ defense mechanisms for an enhanced resistance/tolerance to the upcoming stress, quench reactive oxygen species, have potent antimicrobial as well as allelopathic effects and might be important in regulating plant growth, development and senescence through interactions with plant hormones.
Capturing VOC Power
By optimizing the natural protective VOC response, the potential to combat plant disease without synthetic pesticides is enhanced. A type of “green vaccination” would utilize protective VOCs to prime the crop for optimal response to future stressors. Research has shown that application of VOCs can reduce cold stress in corn, or help wheat plants better resist fungal pathogens.
Companion planting is already widely used, particularly in organic agriculture, as a method to reduce crop pathogens and diseases. Using this technique, plants which emit VOCs that attract or repel certain pests are utilized to keep detrimental insects away from susceptible crops. Planting rosemary near peppers, for example, deters aphids. The VOCs emitted by rosemary discourage aphids from reproducing anywhere nearby. African and French marigold, geranium, lavender, basil, chives and pot marigold also emit aphid-repelling VOCs.
Novel companion planting strategies could utilize modern knowledge of VOCs and the ability to detect their presence. Emission of VOCs by wounded plants has been shown to enhance VOC production of nearby plants, priming them to resist stressors. Planting high-VOC emitting cultivars, or even wild crops which have thrived in adverse conditions, might be a way to prime crops to respond more robustly to threats. These high-VOC emitters, planted near a cash crop, would emit VOCs in the face of adversity, priming the crop to do so as well, prior to an actual threat reaching them.
Since VOCs are known to prime plants against stressors, their direct application in the field would appear to be a positive, natural method to enhance plant health and reduce disease pressures. While VOCs have been identified and utilized in vitro to show antimicrobial activity, field application poses some problems.
VOCs are readily dissipated in the air. They work in conjunction with each other, so the exact mechanisms of action are not yet known. In some cases, it’s been shown that direct application of isolated VOCs can also have unexpected deleterious effects.
Research is continuing on VOCs, including their use as a mechanism to prime the crop to respond more quickly and effectively to disease threats. This effect, known as systemic acquired resistance (SAR), is associated with plant cells’ ability to rapidly release protective proteins when faced with a challenge, much like our immune system response.
Measuring VOC Response
Because the VOCs being emitted can be indicative of plant stressors, measurement of these compounds in the field could help diagnose plant disease concerns prior to symptom appearance.
Diseases have their own signature VOC profiles, and researchers at North Carolina State University have used those profiles to detect and diagnose disease. Their prototype handheld device measures the crop’s VOC emissions.
Much quicker than laboratory tests such as molecular assays, mobile VOC detection will allow farmers to rapidly test plant tissue and analyze VOC profiles. A piece of leaf is put into a test tube in the field and sealed. After 15 minutes, the VOCs emitted are pumped into the mobile detector, which is plugged into a smartphone. This device contains a paper strip of chemical reagents. As the VOCs react with the reagents, a color pattern occurs. This pattern can then be correlated with a given disease’s profile.
The research thus far has detected disease two days after inoculation. It has also discriminated between three similar tomato fungal diseases accurately. The technology, ready to be scaled-up, is anticipated to be further developed to be crop and region specific, and to have automated pattern recognition.
Along with advances in biopesticides, further research into the use of VOCs as crop protectants, as well as disease detectors, should soon provide growers additional natural options for combating pests and diseases in crops.