There's another factor here as well. All fungi require oxygen to grow (but root rots require less oxygen concentration than their competitors). But when the roots aren't getting enough oxygen, they are stressed and release additional exudates. It may well be that these exudates are the ones the disease-causing fungi grow fastest on, on top of all the other conditions selecting for these diseases.

Waterlogged conditions can result in limited oxygen diffusion, so these disease-causing organisms are able to out-compete the beneficial organisms when the soil is too wet.

What is the Foodweb Solution? Tie-up the excess nitrate in the bacteria and fungi. When fertilizer is applied, the excess N will leach from the soil unless it is retained. What retains nutrients in soil? Organisms and soil organic matter (SOM). What's SOM - the metabolic products of bacteria, fungi, microarthropods and earthworms chewing on plant residues. Microarthropods and earthworms break up plant debris into littler and littler chunks, an extremely important process that gives beneficial bacteria and fungi new surface areas to grow on. The waste products from all these organisms, and the not-yet completely decomposed plant material, is SOM. The more organisms, the more SOM accumulates more rapidly. But we need to tie-up the excess fertilizer N in the bacteria and fungi, and on SOM surfaces. Without nitrate, the disease-causing fungi will not be able to outcompete and escape the beneficial organisms.

The second thing is to open up the soil to increase oxygen flow into the soil. When you plow or rip the soil, or when you "aerate" by pulling plugs from the greens for example, you get some oxygen back into the soil. But the soils won't remain "fluffed" unless you get life back into the soil. You have to let them build the structure in soil. If pesticide applications killed many of the organisms, you can't build soil aggregates, and you can't improve oxygen diffusion into the soil. Greens made with sand and little organic matter have few foods to grow the bacteria that make the smallest size soil aggregates, and few foods for fungi that bind the smaller aggregates into visible aggregates. There are no foods for protozoa, beneficial nematodes or microarthropods that engineer soil pores. In just a few years, what little organic matter was present is used up, and the sand compacts like concrete.

To open up the spaces between particles, you need to inoculate the beneficials, and provide food. Soil structure can begin to come back within days as the bacteria and fungi grow. But be careful - too rapid growth of bacteria and even fungi in some cases can suck up all the N, leading to yellow or dying plants. In these just-beginning to-build conditions, the bacteria can use up all the oxygen, and result in anaerobic zones. Balance is critical in the conversion from straight sand to organic matter plus sand. We have work to do to discover the optimal way in all different kinds of conditions to move back into a condition of health.