Insect and disease attraction to plants with reducing sugars
How is it possible for a high Brix plant to be resistant to insects and not provide them with an abundant food source when insects are attracted to sugars? The key insight is that plants contain different concentrations of different carbohydrates at various levels of plant health. The goal for optimal plant health is to have all photosynthates and soluble sugars such as glucose and fructose converted to non-reducing sugars in each 24-hour photoperiod. This means a healthy plant will have a high Brix concentration and very low levels of reducing sugars.
From the podcast interview with Don Huber.
John: Are there any negative health consequences of plants having high levels of fructose and glucose?
Don: Yes and no, depending on what other stresses there are present. If you have a deficiency of manganese, for instance, it can’t store the reducing sugars―glucose and fructose―that are being produced through photosynthesis. It can’t store them as sucrose, and so they become very attractive reducing sugars, and they become very attractive to insect pests and to a number of plant pathogens.
Manganese is a critical factor for that sucrose-phosphate synthase enzyme that converts glucose and fructose into sucrose for storage. If you’re deficient in manganese, you’ll have high reducing sugars―glucose and fructose. As insects like aphids fly over these plants, they can detect that high reducing sugar, and for them, it’s a red flag saying, “Hey, come in for dinner!” But if those sugars are converted to sucrose and stored there, you don’t see that attraction.
Reducing sugars come out of the root system―they’re the root exudates that are attracting Pythium and Phytophthora and Aphanomyces and those other oomycete pathogens―root-rotting pathogens.
John: Don, you described how the carbohydrate profile can attract aphids. Are there other insects that can be attracted by the carbohydrate profile?
Don: A lot of them are. I don’t know that all of them are, but many recognize the difference between the reducing sugars, and they don’t seem to be attracted to the non-reducing sugars nearly as much. You’ll see that association. When we get the minerals balanced for the plant, you’ll see all of those problems start to disappear or be very minor.
P.S. I appeared as a guest on The Modern Acre podcast in this episode.