Downy Mildew: Cold weather disease

1. Why is downy mildew a particular problem this winter?
The fungal disease is favored by typical winter conditions which include cold temperatures, lower light and lower growth rates. Watch for downy mildew all winter and don’t relax until summer is in full swing. The disease has already been seen on rose, snapdragon, alyssum and stock in some parts of the country. The crops most commonly affected by downy mildew include: alyssum, stock, basil, coleus, pansy, viola, rose, salvia, lamium and snapdragon.

2. So is downy mildew one of the worst diseases that growers face?
Downy mildew can escape attention until it is full-blown, at which time many controls can fail. Not all downy mildew is caused by the same fungus. Most plants have their own special form of this pathogen.

3. What are the symptoms of downy mildew?
Symptoms include yellow or reddish leaf spots (blotchy or angular), distortion of new growth (might be a systemic infection) and white, lavender and purple sporulation on undersides. The sporulation is often confined between leaf veins which leads to the blotchy appearance.

In some cases the flower spots (on rose) can easily be confused with Botrytis on flowers. On roses, downy mildew also can cause stem infections which appear as tiny red spots.

Leaf drop overnight can be a clue that downy mildew is involved. This is common on some roses.

4. And the conditions that promote downy mildew?
The conditions that promote downy mildew are very similar to those that promote Botrytis. High humidity and more than six hours of leaf wetness make downy mildew worse.

The optimal temperatures are wide for some downy mildew pathogens (snapdragon downy mildew), narrower for others (pansy downy mildew) and even relatively warm (coleus and salvia downy mildews). In general, the optimal temperature range is between 50°F-75°F (same as Botrytis).

Poor air circulation which can occur in the closed winter greenhouse leads to very rapid downy mildew outbreaks. It is critical to never water late in the day, extend mist into the night unless absolutely necessary or spray fungicides more than twice a week.  Each of these activities can result in immediate outbreaks.

5. What are common sources of downy mildew?
Downy mildew can be seed-borne (occurs sometimes on snapdragons), but is also started by using infected seedlings or cuttings. I have opened bags of unrooted cuttings from offshore suppliers that had active infections and sporulation of downy mildew (coleus).

Quick control tips:
Dimethomorph (Stature SC) is especially effective for controlling downy mildew. Strobilurins (i.e., Heritage, Compass O, Pageant and Insignia) are also very good choices.

Fenamidone (FenStop) is not a strobilurin, but should not be rotated with a strobilurin due to potential for cross-resistance. Phosphonates (i.e., Aliette) remain very good choices for downy mildew control. Subdue MAXX has been used for years to eradicate downy mildew and is now legal for this use on ornamentals.

Adorn provides very good control and it must be mixed with another downy mildew fungicide when used for this disease. 
 

FYI:

• Get out in the greenhouse and look closely for early signs of downy mildew. Look under the leaves. Water plants early in the day. Don’t allow the plants to exceed the six hour leaf wetness minimum or the disease can explode.
   
• Don’t use a copper fungicide when it won’t dry quickly. Don’t spray flowers with chlorothalonil and try to resist spraying more than once a week. Be sure to rotate modes of action since downy mildew populations have proven resistant to some fungicides. 
   

Have a question? You can write Ann at archase@chaseresearch.net.