Question
Is it safe to use on plants? I have a nectarine with gummosis and I'm wondering if I can apply h2o2 to kill the fungus. Or will it hurt the tree?

Answer
Gummosis is a symptom of another underlying fungal infestation or bacterial infection.  Without treating the cause of the gummosis, you are just delaying the inevitable.  Hydrogen peroxide functions at the cellular level by producing hydroxide radicals which are destructive to fungal, bacterial, and plant cells (animal cells, too).  During normal functioning, cells produce a number of oxygen radicals including hydroxide radicals which specialized enzymes isolate and detoxify before eliminating as waste products.  So, in theory, treatment with hydrogen peroxide should work to control the fungal infestation.  The problem is that these infestations are usually located not on the bark but between the bark and the sapwood.  Application of fungicides to these parts of the tree is difficult, so the usual control measures are to prune affected branches.  To answer your question specifically, I doubt that the hydrogen peroxide would affect the tree because of the physical and chemical barriers produced by the plant.      

Gummosis in stone fruit is usually caused by _Botryosphaeria dothidea_, and affected trees may recover from mild infestations.  The fungi that produce these symptoms are most easily controlled by removal of affected branches. If you have confirmed that the disease is fungal related, you can follow some of the following guidelines for control.  

Disease control must be based on several practices; no one practice is effective by itself. The following measures help indirectly by reducing ports of entry for the canker
fungi, by moderating tree stress, and by reducing inoculum levels of the fungi. Each of these factors must be considered before adequate canker control (gummosis) can be attained in any given tree or orchard.

1. Delay pruning activities until growth starts in the spring since callus formation starts quickly at this time.
Fall pruning can severely weaken and stress trees and can set them up for winter injury and subsequent infection.
2. To promote rapid wound healing, prune branches to leave the swollen collar at its base. Do not leave stubs and avoid a very close flush cut. Do not leave weak-angled crotches when shaping trees as these are potential
sites of infection.
3. Practice good sanitation.  Prune out and destroy (e.g. burn or bury) badly cankered limbs.
4. Fertilize trees according to soil test recommendations. Avoid late summer fertilization.
5. Avoid unnecessary injury and control insect pests, such as peach tree borer and lesser peach tree borer.
6. Water trees during dry weather to reduce stress.

This may seem like I am dodging the question, but I think hydrogen peroxide could be costly and ineffective.  Hydrogen peroxide is usually sold at a concentration of 3%, and I am not certain that would be an effective concentration to accomplish fungal control.

Good Luck and Good Gardening.