Michelle Grabowski, University of Minnesota Extension Educator
As the season warms up and new leaves reach their full size a few gardeners are noticing unsightly spots in a wide variety of plants.
What are these spots?
Many of the spots that gardeners are seeing are caused by fungal or bacterial pathogens infecting the leaf tissue. There are a few look a likes, however, so gardeners should pay attention to the details. Fungal and bacterial leaf spots are typically randomly scattered across the leaf. There are often several sizes of leaf spots because the spots get bigger as the pathogen grows. In addition leaf spots caused by a pathogen start out on one leaf and eventually spread throughout the plant or to other plants. A gardener may notice fluffy white cobweb like fungal growth, powdery spores or other signs of the pathogen in the spots themselves.
Photo 1: Notice the random pattern of the leaf spots and the different sizes of spots caused by this fungal leaf spot disease or iris. Michelle Grabowski
Spots that are all one size, do not grow and spread, or occur in a specific pattern are not likely caused by fungal or bacterial pathogens. Many gardeners confuse leaf damage caused by fourlined plant bug with a fungal leaf spot disease. See the article, Fourlined Plant Bugs Are Now Active, in this issue for more information about fourlined plant bug. Notice how the spots caused by fourlined plant bug are all the same size.
Photo 2: This Munstead lavender is for sale and comes with a free fungal leaf spot disease! Michelle Grabowski
Where did these spots come from?
Fungal and bacterial leaf spot pathogens can come into the garden from a variety of places. Some are blown in on the wind. Others come in on infected plants. Most survive from season to season in last years infect leaves. Splashing rain or spring winds carry the pathogen onto the newly emerging leaves.
What to worry about?
Leaf spot diseases are not a serious threat to the health of the plant if they do not result in major leaf loss or discoloration. If the leaves are mostly green and still attached to the plant, they can still undergo photosynthesis and provide food for the plant despite a few spots. Many leaf spot diseases fall into this category and require no management.
Photo 3: This plant of Rosa foetida 'Austrian Copper' has been seriously defoliated by black spot, a fungal leaf spot disease. Michelle Grabowski
Leaf spot diseases that result in defoliation or discoloration of the majority of the leaves should be taken more seriously. For example black spot on rose can seriously injure a plant because in addition to spots, the black spot fungus causes leaves to turn yellow and fall off. This reduces the energy available to the plant and can result in reduce growth, flowering and winter survival. Gardeners should consider using a combination of management strategies to slow the spread of these pathogens.
What can you do?
There are several basic practices that a gardener can implement to reduce the amount of leaf spots that occur in the garden. Most fungal and bacterial pathogens need moisture or very high humidity on the leaf surface in order to start a new infection and to produce new spores or bacteria. Many management strategies focus on reducing leaf wetness.
Photo 4: Powdery mildew on phlox can be very severe. Plants that are seriously infected each year should be replaced with a resistant variety. Michelle Grabowski
- Tolerate leaf spots if the majority of the leaf is green and no major leaf loss is occurring.
- Use drip irrigation or a soaker hose to water plants instead of sprinkler irrigation.
- If sprinkler irrigation is the only option, water in the morning so that leaves dry quickly in the sun.
- Space plants for good air movement in the garden. This will help leaves dry quickly after rain or dew.
- Split dense, overgrown perennials to improve air movement through the garden.
- Mulch garden plants to reduce humidity.
- Do not work in the garden when the leaves are wet. Wait until plants have dried to prevent accidentally moving the pathogen on your hands or tools.
- Remove last year’s plant debris from the garden.
- If only a few leaves are infected, pinch off infected leaves and remove them from the garden. Never remove more than 1/3rd of a plants leaves or you will do more harm than the pathogen!
- If a plant is severely infected, look for a disease resistant variety to replace it.
- If all of the management strategies above do not reduce the leaf spot disease to an acceptable level, consider protecting plants with a fungicide. Always read and follow all instructions on the label any time you apply a pesticide!