Blackspot: What is it & treatment

 

Rose leaf diseases: Blackspot

What is it? 

Blackspot (Diplocarpon Rosae) is the arch nemesis of rose lovers. It's a fungus that can defoliage an entire plant. Vulnerability to Blackspot on cultivated roses came into this world when the  classic European roses got crossed with the reblooming rosa Chinensis or China rose. 

This means that classical rose groups which flower only once won't get blackspot. Bourbons, Portlands, Hybrid Perpetuals and Hybrid teas are all susceptible to it. If it reblooms its susceptible. When the HT Soleil d'or Was created she opened the door to many bright colors but she was extremely susceptible to Blackspot so the genes of Soleil d'or are in nearly all modern roses. 

The last decade there's been a lot of research on the Diplocarpon Rosae. The rose industry is really big and Blackspot is the main problem.

This fungus has the ability to morph into another strain, just like the flu or Covid. This means that resistance to Blackspot isn't permanent, not even with the new Varieties. They can withstand it for a while but eventually they'll get infected by a new strain. It's like the Star Trek Borg collective: Weapons can destroy them at first but they adapt soon and become invulnerable to the weapons.

Research has shown that infection on the plant spreads by water. The fungi spores are airborn and can travel several km or miles. So even if your garden is free of infected leafs and spores they can get infected by air. Once the spore land on the leafs of roses (transported by rain drops) it needs seven hours to infect the leaf. So it needs a wet environment for seven hours in order to survive. If the leaf gets dry earlier it won't get infected. But if it stays wet long enough the infection is permanent. Spreading to other leafs occurs when water falls onto infected leafs and drizzles onto other uninfected ones. But this can also only occur if it stays wet for seven hours.

So now we know that:

It's a fungus and it spreads by spores which are airborn. It can mutate to another strain and infect so called resistant varieties. Rain drops are its ride to land onto our roses and it needs seven hours of wet conditions to infect the leaf.

Cure and prevention: Curing the infected rose isn't that difficult. Very good results are reached with a treatment of Neem oil or whole fat cow milk (1:3 so 1 part water mixed with 3 parts milk). There are other bee friendly products on the market by different brands so you can find them online and shop there. Don't use regular chemical fungicides because it destroys the biodiversity of your garden, it's poison.

When using Neem or cow milk repeat weekly on infected plants for at least one month. As a prevention: Start spraying when leafs appear (so now) and repeat every two weeks until September. These products strengthen the plant without causing damage to other organisms.

When you detect the disease on a leaf or leafs remove it immediately and burn it. Start to spray with Neem or another bio product immediately and also treat roses next to the infected one.

In Autumn: When leafs have fallen down you can collect and burn them. Don't put them on the compost pile for obvious reasons. A thick layer of mulch prevents infection also So mulch on top of the fallen leafs if you don't want to collect them. Spray the naked rose canes with Neem or your product of choice. Now your roses are safe and Blackspot free.

Don't plant roses too close together. It helps to spread the infection and it prevents the leafs to dry up soon after rain. Also with Blackspot; Don't water the roses directly onto the leafs. Give water on the foot of the rose.

Practice in the UK and Holland has shown that roses planted with Mycorhizza are less likely infected with blackspot. The Mycorhizza spores are kind of natural antibiotics it seems.

So that's it people. So never despair, Blackspot can occur on all reblooming roses but with these good tips it can be prevented and cured as much as possible.