Red spots and holes in Hydrangea


TXGardener

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Hi folks, me again. Can someone please help me identify and eradicate this disease on my recently transferred (pot to ground) Hydrangea?

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That is an unfortunately common complaint this year. Had it on mine as well. Turns out it is possible the most common fungal attack upon hydrangea no doubt helped along with humidity in your area?

The rounder spots are usually Cercospora.

If they are triagular they may be a bacteria Xanthomonas.

A systemic fungicide is useful. Agri-fos is is helpful, but the plant will survive without treatment and the wounded leaves will not recover. Its not bad yet.

Other fungicides work as well. Agri-fos has other names. It is salts of phosporous acid and some consider it organic. Here you see it has no toxic elements
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There are a few things that may help, though they are hardly guaranteed preventatives.

The most important is sanitation. Remove all the fallen leaves and much under your Hydrangea in late Fall with a clean woodchip or bark mulch.

Also avoiding overhead irrigation may help, though of course, there isn't much you can do about rain splashing the leaves.
 

NigelJ

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Just a couple of points about phosphate, phosphoric acid, and phosphorus.
Phosphoric acid is found in Coca Cola and and is why it can be used to clean coins and as a gentle rust remover (I know it needs a good rinse to get rid of the sugar)
Phosphorus itself can be toxic (search for phossy jaw). It is also found in phosphine (PH3) a very toxic gas used as a pesticide and organo phosphorus pesticides and nerve agents.
Phosphate itself has a finite supply on this planet, it is also responsible, along with nitrate, for algal blooms in water courses so shouldn't be over used.
Some Australian and South African plants are very sensitive to phosphate.
 
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Just a couple of points about phosphate, phosphoric acid, and phosphorus.
Phosphoric acid is found in Coca Cola and and is why it can be used to clean coins and as a gentle rust remover (I know it needs a good rinse to get rid of the sugar)
Phosphorus itself can be toxic (search for phossy jaw). It is also found in phosphine (PH3) a very toxic gas used as a pesticide and organo phosphorus pesticides and nerve agents.
Phosphate itself has a finite supply on this planet, it is also responsible, along with nitrate, for algal blooms in water courses so shouldn't be over used.
Some Australian and South African plants are very sensitive to phosphate.
This link from Penn State separates a lot detail for more details. https://extension.psu.edu/understanding-the-phosphonate-products


In this agri-fos case, another (4th?) is used called phosphorous acid not phosphorus. Evidently the plants think it similiar to the nutrient and take it inside but it is not used in the normal ATP energy cycle. I am interested to learn more about the fungicide and how that works, but the various plant pathways are a maze to me.
 
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