Hydrogen Peroxide is H2O2, two hydrogen atoms and 2 oxygen atoms bonded together. It has one more oxygen atom than water, which is H2O. Studies have shown that plants create H2O2 and similar molecules in response to fungal and bacterial attacks. You can keyword search: The Oxidative Burst in Plant Disease Resistance for detailed information. 

This an extremely simplified explanation of how Hydrogen Peroxide or H2O2 works to kill fungi and bacteria on your tomato plants. The bonds that form a molecule of hydrogen peroxide are very unstable.