Yellow Spots on Leaf - Downy Mildew - Causes and Treatment

It's very frustrating when, after planting and caring for a plant for many years. Yellow, pale-green, or angular spots suddenly appear on the upper surface of the leaves. If you see this on your plant, you know it has Downy Mildew.

In this article, I will give you what I did when this happened with my plants, why this happened, and what practices you should follow to prevent this from happening to your plants.

Commonly Affected Crops

Downy Mildew is very common on Grapes, Cucurbits (cucumber, melon, pumpkin, squash), Lettuce, Spinach, Basil, Brassicas (cabbage, broccoli, cauliflower), and Ornamentals (rose, impatiens, etc).

Downy Mildew is a very common plant disease worldwide, but especially in regions with humid, wet, or cool-to-mild climates.

• Greenhouses (high humidity, poor airflow).

• Cool, wet seasons outdoors (spring, fall).

• Tropical/subtropical areas with frequent rains.

Causes And Symptoms

It looks like a fungal infection, though it's not caused by a true fungus. It’s caused by oomycetes (water molds) — organisms in the group Oomycota.

Several symptoms that you may notice:

• Yellow, pale-green, or angular spots on the upper surface.
• A fluffy, grayish-white to purplish “downy” growth on the undersides.
• Young leaves, shoots, or flowers may be distorted.
• Premature leaf drop, reduced photosynthesis, and yield loss.

What can you do to save the plant from this?

One thing to keep in mind: The treatment will be slightly different for different crops because:

Each crop has a different pathogen species. You may understand this if I give a data-based example: Plasmopara viticola infects grapes, Peronospora effusa infects spinach, Bremia lactucae infects lettuce. Although all those are oomycetes (water molds).

So if a fungicide is effective on one crop that does not mean it will also be approved or effective on another.

What to do to prevent Downy Mildew?

When you first plant a crop, you can spread a small amount of sulfur on the soil, which greatly reduces the chances of Downy Mildew.

I would also like to mention some compositions that you can use.

• Metalaxyl-M at 4% and Mancozeb at 64%
• Cymoxanil 8% and Mancozeb 64%
• Iprovalicarb at 5.5% w/w and Propineb at 61.25% w/w

Mix approximately 1½ teaspoons of product (any one composition) per gallon of water and spray the plant once a week.

What to Do if Your Plants Get Downy Mildew?

If, despite all this, your plant still gets Downy Mildew, you can use these compositions.

• Metiram 55% and Pyraclostrobin 5%. Use about ⅙ teaspoon of this composition per gallon of water.
• Or Kresoxim-Methyl 15% and Chlorothalonil 56%.Use about 1½ teaspoons of this composition per gallon of water

I have been using these for the past few years, and they work for me, as well as my neighbors.