You're awfully optimistic. Who says that a fungus that makes diesel fuel won't just encourage people to chop down more rainforest so they can make diesel fuel out of it?
Why take the effort to chop down trees to make fuel when we already produce enough cellulose from crops (corn stalks, sawdust etc) to make a lot of biofuel? It would be more cost effective to use sawdust from wood mills, corn stalks, etc (things normally seen as waste materials) to make fuel than it would be to go out and chop down trees. Not only would you have to expend a lot of energy to chop down the trees and transport them, but you would have to process them into a more usable form. I doubt it would work well to simply coat an entire tree with a fungus and wait for it to be decomposed. You would want to grind the tree up into smaller bits to increase the surface area the fungus has to grow on .
Assuming you have sawdust and corn stalks. If you're in a part of the world where rainforest is more plentiful than either of those, and you're chopping it down anyway to make farmland...
