Starch is not soluble in water. Glucose is the fuel for life. When glucose reserves are high, defense is high. When glucose reserves are low, defense is low. When defense is high, we say the organism is healthy.
Most pathogens are opportunists and wait until defense is low before they attack.
Before I continue with this subject, I should give some other characteristics of root wood. Roots also mechanically support the tree. Root wood does have lignin, but not as much as trunk wood. This is best shown when you cut trunk wood and root wood of the same tree and feel the difference. It is much easier to cut root wood than trunk wood.