Since we just finished talking about roses last week, I thought I’d discuss one of their most disfiguring problems this week since all this week I’ll be discussing what happens to plants when we get hot and humid weather. As you can imagine, my approach to all of this is rather hands off, but some folks do like to treat.
Two things I’ll tell you about fungal diseases–they are easier to treat before they are a problem. So if you have them this year, plan to treat proactively next year. Next, fungicides are much better at preventing problems that they are at curing them. Once you have the fungus all they can do is manage it.
I’ll also tell you that it is a rare fungus that will kill a plant. Sometimes you will wish that the plant will die because it gets so ugly–but that’s a different thing.
Blackspot can be prevented in roses by giving them good air circulation. You can see that I break that rule and so I’ll have issues. This David Austin™ rose is underplanted with both a smaller Fairy rose and with catmint. The catmint deters the japanese beetles, but it runs amok in the garden and probably causes moisture to linger, hence the blackspot.
What do I do? Nothing. When the leaves get too diseased, I pull them off and throw them in the trash.
Should you wish to be a little more proactive than I am, there are lots of good organic remedies that are easy on the environment. Neem oil is said to be a fairly decent fungicide in addition to being a pesticide.
If you’d prefer to use something less toxic than that, a 50%/50% mixture of milk and water is always a good remedy. Just be sure to clean out your sprayer after every use.
I’ve heard that liquid seaweed or liquid kelp sprayed on the leaves is also a good preventative but I have not tried it myself.
And finally there are organic fungicides on the market, but they do vary in toxicity to bees and other beneficials so be sure to investigate those before using, even proactively.
Gardendaze 
I, too, ignore blackspot when it strikes. Not crazy about how it looks but I seldom have a problem with it. I’ve heard baking soda is also a good remedy.
Wendy,
I’ve heard about baking soda too but haven’t tried it. I have a remedy called GreenCure which is potassium bicarbonate (if I’m recalling correctly) instead of the sodium bicarbonate of baking soda. It does work–but I’m still a believer in the good old milk and water if I have to use anything. It’s something I always have on hand because “The Spoiler” uses milk for his coffee–and it’s something I don’t object to eating if I have to use it on an herb like rosemary.
But I was out there this morning taking off a few of the worst spotted leaves–sometimes that’s just as good a remedy as anything.
Karla
Doesn’t the milk get stinky in the sun? I have visions of curds and whey and Little Miss Muffet and a spider pinching its nose.
Wendy,
You’d think it would but it doesn’t seem to–and I’m not at all scientific with my proportions of milk and water. I suspect I’ve gone as high as 80% milk and 20% water. Now of course I only spray early in the morning or after the heat of the day–but that doesn’t mean that the stuff isn’t there the next day baking in the sun.
I can’t say I’ve ever tried it for the black spot but I have used it for a few of the other fungi I’ll be posting about this week particularly powdery mildew on stuff like herbs and I tried it on some tomato blights and on that awful blight that the basil gets (I can’t recall if it worked on that or not).
I’ve also used it after I’ve cut down my peonies when they get that black botrytis to sanitize my peony cages and stuff–and to spray on any good foliage that was left.
But no curds or whey or even greek yogurt.
Karla