Look Out for Downy Mildew

Look out for Downy Mildew

Downy Mildew on the hybrid tea Sacred Heart, April 2018

Danger. Danger. Danger. Downy Mildew is a disease of many plants particularly vining ones like grapes, cucumbers, melons, and squash but it also will affect ornamental plants including roses.

The weather in Salinas has been particularly unusual in 2018. The rains did not start in earnest until March. By then, most gardens had been pruned weeks before and leaves were already out. When the rains began, temperatures dropped and the leaves were often wet for long periods of time. With ever increasing sunshine and continued low temperatures, the ideal conditions for downy mildew were in place.

One of my new roses, a hybrid tea named Sacred Heart, came from a local nursery. It was in the reduced-for-quick-//sale section of the parking lot and had been at the nursery in all likelihood for at least 12 months. That's usually a plus as such roses are eager to have a new pot and fresh soil and respond well when they get them.

Like all other new plants in my garden Sacred Heart was potted up, pruned a bit with all old leaves removed. I hadn't sprayed the pruned canes with a chlorine solution or hydrogen peroxide but trusted things would be fine. Apparently, it brought many spores of downy mildew with it.

Just last week, I noticed mysterious blotches on its new leaves. I've had plenty of experience with the three common fungal diseases in roses: black spot, rust, and powdery mildew. But these blotches were different.

My first diagnosis was sunburn. It can happen to leaves and canes which have been growing in the shade (very cloudy weather even) and suddenly the April sunshine beams down. Reason prevailed. Why did Sacred Heart get sunburned when the other new roses around it did not?

I reached out to fellow rosarians for help and the answer came in: downy mildew.

All agreed the weather this season has been unusual. Winter came late and wet and often days went by when the rose leaves never completely dried off and the temperatures were often highs around 60 degrees and lows in the 40s.

Other reported effects on roses were short stems on the hybrid teas, many blind shoots (new canes with no buds at the top), and "high rose tide" delays by at least two weeks.

Downy mildew on a hybrid musk showing its stoppage at a vein

Details on Downy Mildew

Unlike the true fungal diseases mentioned above, downy mildew is caused by an organism more closely related to algae. Therefore, fungal sprays won't have any affect on curtailing it. The disease isn't all that common along the Central Coast because we usually dry out by the time leaves are in profusion.

Its spores are airborne and usually associated with poor sanitation and lots of water. It's an algae so think wet: wet infected leaves, stems and soil.

Also different from the fungal diseases, downy mildew is a pathogen. If left untreated, it will kill the plant. It is parasitic.

Jolene Adams, former president of the American Rose Society and a resident of the East Bay, responded with the following comments:

"One of the "defining" characteristics of Downy is that it does not cross the leaf veins.  So - in your photo you can see the blotches of the mildew stop at a vein and start on the other side.

People do get it confused with BS because it is often on the same leaf at the same time.  And its blotches can be oval or circular - but they still do not go into a vein.

Baldo (Villegas, an environmental research scientist for California’s Food and Agriculture Department and a noted expert of plant pests and diseases) taught us - long, long ago - to turn the leaf over, pinch it closed so the mid-vein stands up and then look closely at the blotch close to it with a 10x or better lens.  You'll just barely be able to see tiny fuzzy growth rising from the leaf surface.

My eyes are so bad now I don't bother anymore. I just de-leaf."

The best control involves isolating the plant immediately if it's in a pot. If that's not possible, pull off all infected leaves and spray what's left with a mild solution of a tablespoon of bleach to a quart of water. If you halt its progress (and the weather warms up again), you will most likely get it under control. As a further safeguard, remember to remove all leaf matter at next season's garden clean up.

Safest way to control downy mildew. Water well and leaves will reappear.