And would you guarantee that the large amount of petroleum, and the methane GAS that is escaping the well, along with the dispersant as it breaks down, cannot evaporate into the atmosphere and 'come down with the rain' like the gases you mention?
I'm not asking you to judge whether they are what is causing this crop damage, but whether the chemicals (as gases) from the sources mentioned could evaporate and come back down with the rain.
The dispersant isn't breaking down all the hydrocarbons escaping the well, just the oil. The various hydrocarbons are separating from each other themselves. Even without the application of dispersant, the natural gas - and by some estimates, that accounts for at least half of the volume of material escaping the well head - will eventually rise to the surface and immediately evaporate because natural gas is just that - a gas in its natural state. The dispersants do nothing to hasten this process.
Now, if you were to ask me if the non-oil hydrocarbons, many of which have a high natural volatility - pose an environmental risk and could be carried inland by air currents and deposited on land mixed with rain water, the answer to that is yes. I would guess - although I'm no expert in the field - that because we've had other huge leaks, both in ocean and surface wells - if such atmospheric contamination posed a threat to plant-life, we would have already been aware of such a phenomenon, at least any large scale phenomenon.