When plants become stressed through lack of nutrients, over- or under-watering, wrong growing conditions (exposure, poor pH levels, etc), injury, overuse of toxic fertilizers and any number of conditions that impact on their health, they give off stress hormones. These are very targeted signals to predator insects and disease microbes (pathogens). Insects or pathogen interprets these signals as food,
These predators are very host-specific, meaning that most insects and microbes feed only on a very select group of plants. These predators are not necessarily the bad guys, but play the role of nature’s clean-up crew like other animals and birds that remove the sick and diseased.
There are a number of ways nature deals with pest and disease attacks:
1) Leaves: All plant leaf surfaces are naturally colonized by a great diversity of microbes that feed on the metabolic wastes excreted by plants.
In exchange for carbohydrates, they perform a variety of services for the plant, such as out-competing organisms that might feed on plant leaves; producing toxins that kill plant-feeding microbes or insects; producing plant growth hormones and fixing nitrogen, which is absorbed by plant leaves; and even degrading airborne pollutants that have collected on the plant surface.
Endophytic fungi have a symbiotic, or co-operative association with plants by living right inside plant leaves, and interact with the chemistry of the plant to produce compounds that are toxic to other organisms, particularly insects and mammals.
2) Soil: In healthy ecosystems where organic matter is constantly recycled, plant roots are colonized by special types of fungi, called mycorrizhal fungi, which serve as a first line of defence.
This fungi anchors themselves to the plant roots and then sends out literally miles of tube-like hyphae to scavenge the soil for nutrients that are difficult for plants to obtain, and in return, trade for carbohydrates.
In healthy plants, the vulnerable young roots are completely surrounded by these fungi, which will attack and prevent any approaching plant predators from reaching the root surface.
Other plant allies are the myriad of microorganisms inhabiting the area in the direct vicinity of plant roots, where they feed on the plant’s metabolic wastes, sloughed off root cells and other carbon compounds excreted by plants. Just like the leaf surface dwellers, these organisms out-compete plant pathogens and produce a variety of defensive compounds.
3) Plant communication: Plants can communicate with each other through chemical signals and through the network of fungi in the soil. When they receive signals from other plants that are under attack, they respond by manufacturing their own insect or microbe specific defensive compounds to gear up for any attack.
Tilling, improper watering methods and compacted soil will disrupt these fungal aerobes line of defence and ability to communicate between plants.
Always assess the conditions the plant is growing under and eliminate or address the underlying issues before you reach for the often poisonous bug control and disease remedies.
Even the most gentle insecticidal soaps often create problems because they’re not species-specific and they damage the plants’ natural ability to ward off any threats.
We need to support plant health by giving them the best growing conditions possible and boosting their defences by colonizing the beneficial microbe populations through foliar spraying or adding them to the soil through organic matter or sprinkling them in. See The Organic Gadener’s Pantry in Victoria or try cultivating your own.