Blackspot or Downy Mildew?

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Blackspot

I get emails from many rose growers in spring and fall who think they have blackspot.   Many of them have been treating for this disease and don’t understand why the fungicides they are using are not working.    In most cases I find they have misdiagnosed their problem and what they actually have is downy mildew.

I will talk about how to control both diseases in future blogs, but for today let’s focus on what they look like.

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Advanced stages of blackspot

Blackspot (Diplocarpon rosae) starts as a small black spot/spots on otherwise green healthy leaves.   As the disease progresses, the leaves get more spots and the green tissue begins to be replaced with yellow.  Usually within two weeks of infection the leaves will begin to fall and the entire plant may be defoliated.  Left untreated the plant will try to put forth new growth, only to also become infected and again defoliated.

 

I find downy mildew is misdiagnosed for two primary reasons:

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Downy mildew on young growth.

1.  People associate the term “mildew” with something that should look like either powdery mildew or the black mildew on some very wet item or place.

2.  Because the lesions look similar in some cases to blackspot, the grower assumes that’s what it is.

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Downy mildew

Downy mildew (Peronospora destructor) is most prevalent in cool wet weather (spring and fall).   The spores normally produce  at night.  The disease usually starts as purple lesions on new growth.  This can sometimes even look like early stages of powdery mildew before the “powder” has formed.   Once the disease progresses, the leaves will continue to mature with dark purple (not black) lesions and sometimes even dead brown tissue surrounding the lesions.   In advanced stages the canes themselves will take on these purple lesions and at this point the plant is highly infected.  Left untreated downy mildew can destroy your entire rose garden.

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Advanced downy mildew.

We will talk about prevention and eradication in my next blog.

For now, know what to look for!