Phosphorous Acid (Excel LG) vs. Sunflower Downy Mildew

[RayR]

Plasmopara halstedii is a oomycete pathogen that causes Downy Mildew on members of the Sunflower family. This pathogen can reproduce sexually and produces thick-walled oospores that can survive in the soil, even in cold climates.
Plasmopara halstedii can infect a plant in two ways. When conditions are right, the oospores germinate into zoospores and can infect a young seedling systemically through the roots. Airborne zoospores can also infect plants non-systemically through the leaves.

Here’s some informational links about this pathogen:

Downy Mildew of Sunflower in Nebraska

Downy Mildew of Sunflower

Last spring I noticed a volunteer sunflower in my garden that showed the unmistakable signs of systemic infection. I have seen this once before and I pulled the plant because an infected plant would be severely stunted and just plain ugly, would never produce a nice flower and would just be a factory for creating lots of new zoospores and oospores. This year was different; I had recently purchased a bottle of Excel LG (now renamed “Plant Doctor”), one reason for my purchase was to combat Downy Mildew on my cucumber and squash, I never even thought about Downy Mildew on Sunflowers but since I now had a plant with the systemic infection as well as secondary leaf infections on other seedlings in progress I would have to see how Mono-di-potassium salts of Phosphorous Acid would act against it. It is well documented that Phosphorous Acid compounds have a direct effect on many oomycete pathogens by inhibiting their metabolism.

I mixed 1 oz. of the concentrate with 1 gallon of water and filled my pump sprayer and liberally covered the leaves top and bottom. Whatever was left in the sprayer I dumped at the base of the systemically infected plant.
4 days later I went to check on the plants and the white powdery growth was all gone on the bottom of the leaves, those areas were now a grayish brown color (third picture).
I applied the spray once more a week later even though there was no sign of any new infections. The sunflowers grew out normally through the summer, even the systemically infected plant had one side branch coming from the base of the main stem that continued to grow normally and produced a normal flower.