Extension Educator, Horticulture
We whine and whimper about below than average temperatures in March; however for our plants it's been better than the above average temperatures of last year. Any emerged leaves of bulbs may have gotten nipped by the severely cold temperatures but should still flower just fine.
March 2013 has also given us more moisture than last year so as a gardener I am pretty hopeful for a good season. I thought I would start out with some good news. Ok now for the bad news for lovers of garden impatiens.
A new disease of Illinois, downy mildew of impatiens (Plasmopara obducens), is threatening all cultivars of garden impatiens (Impatiens walleriana), according to University of Illinois plant clinic diagnostic outreach specialist Stephanie Porter.
Many plants are susceptible to downy mildew pathogens and are usually not considered to be major disease threats. However downy mildew of impatiens is a different beast as it can quickly kill impatiens.
With downy mildew of impatiens leaves curl downward on newer growth. Soon, white to light-gray fuzz may show on leaf undersides. New leaves may appear as stunted or discolored (yellow or pale green). Unfortunately, this disease can infect very quickly and cause complete leaf defoliation.
According to Porter when this disease first became a problem in the U.S., it was seen only in production nurseries and had not yet established itself in the landscape. In 2011, the UI plant clinic began receiving reports that it was a problem in landscapes in the Chicago area.
Based on the reports of impatiens downy mildew infection and weather conditions in Illinois the last two years, Porter believes that outbreaks are likely to continue in the Chicago area because many landscapers and gardeners plant impatiens in the same area every year. An outbreak of downy mildew one year often means another outbreak the following year.
However, it is more difficult to make predictions about this disease in downstate Illinois. We do know that we have had conditions favorable for downy mildew infections in Illinois in 2011 and 2012.
Suggestions for avoiding and managing downy mildew of impatiens:
As with all plant purchases, when purchasing all garden impatiens (including double impatiens and mini impatiens), balsam impatiens, garden balsam, or rose balsam (New Guinea impatiens are resistant), be sure that they are in good health and inspect them for disease. Native wild impatiens (jewelweed) is also susceptible to impatiens downy mildew.
If you had impatiens that died prematurely last year even with watering during the drought and you remember fuzzy growth on distorted leaves, you may want to plant something different in that area this year.
Don't plant impatiens too closely together or in heavy shade.
Do not water impatiens via overhead sprinklers (especially at night) and avoid any other conditions that may promote leaf wetness. Fungi love wet leaves.
Scout for this disease often, especially during cool temperatures (spring or fall). Early detection is key. Suspected plants can be sent to the UI Plant Clinic.
Gardeners who find downy mildew should remove all of the diseased plants to avoid further infection in the garden. However, it may be difficult to rid the planting area of this disease because the pathogen can remain in the soil.
Remove all diseased plants as soon as possible. All infected plants, fallen debris, and roots should be removed and destroyed. Do not put suspected downy mildew plants in the compost pile.
Once plants are infected with this disease, there is NO chance of saving them.
Fungicides are available; however they must be used as protection before the disease occurs. They will not "cure" this disease. In addition, the fungicides need to be applied often, so they usually are not an economical choice for home gardeners.
For more information on downy mildew of impatiens: http://www.ballhort.com/pdf/ImpatiensDownyMildewGrowerGuidelines.pdf