By Judy O’Mara
I was out working in the garden last weekend and I saw the first appearance of iris leaf spot (Didymellina macrospora). This is a very unattractive fungal disease. The iris planting puts out nice looking flowers each year but for the rest of the season everything looks rough, with heavy leaf spotting and leaf scorch. Iris leaf spot will show up in most years but will be severe in years that are wet.
Iris leaf spot is not an easy disease to clean up because it overwinters in the residue. So the first step for management is to clean up the flower bed in the fall after frost has killed the tops. This will help to reduce the amount of disease that is carried over. Unfortunately it won’t get rid of the disease. If the planting is old and crowded, digging them up and respacing them will improve air flow. This can help to reduce disease severity.
Start fungicide protection (chlorothalonil or mycloblutanil) when leaf spotting first shows up early in the spring. Four to six applications may be needed at 7-10 day intervals. Adding a spreader sticker will help coverage and effectiveness of the treatment.
For more information on iris leaf spot check out the following K-State Horticulture fact sheet. https://hnr.k-state.edu/extension/info-center/common-pest-problems/common-pest-problem-new/Iris%20Leaf%20Spot.pdf
Hey, what are those black spots on those roses?
You might have guessed, it’s “black spot.”
Here is some great information from Ward Upham:
Blackspot of Roses
A common disease of roses is blackspot, a fungus disease that can cause defoliation of susceptible plants. Look for dark, circular lesions with feathery edges on the top surface of the leaves and raised purple spots on young canes. Infected leaves will often yellow between spots and eventually drop.
The infection usually starts on the lower leaves and works its way up the plant. Blackspot is most severe under conditions of high relative humidity (>85%), warm temperatures (75 to 85 degrees F) and six or more hours of leaf wetness. Newly expanding leaves are most vulnerable to infection. The fungus can survive on fallen leaves or canes and is disseminated primarily by splashing water.
Cultural practices are the first line of defense.
1. Don’t plant susceptible roses unless you are willing to use fungicide sprays. For a list
of blackspot resistant varieties, go to: http://www.ppdl.purdue.edu/ppdl/weeklypics/3-22-04.html
2. Keep irrigation water off the foliage. Drip irrigation works well with roses.
3. Plant roses in sun in areas with good air movement to limit the amount of time the
foliage is wet.
4. Remove diseased leaves that have fallen and prune out infected rose canes to minimize inoculum.
If needed, protect foliage with a regular spray program (10- to 14-day schedule) ) of effective fungicides. Recommended fungicides include tebuconazole (Bayer Disease Control for Roses,Flowers and Shrubs), myclobutanil (Immunox, Immunox Plus), triticonazole (Ortho Rose & Flower Disease Control) and chlorothalonil (Broad Spectrum Fungicide, Garden Disease Control, others).
Here is a closer zoom of some of the spots.
If you really look closely (or, in real-life, use a hand-lens) you might see tiny little bumps. Those bumps are the structures where the fungus produces its spores. Zooming in closer, with the help of a compound microscope, we can see the individual spores. Each spore can cause a new infection.