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Slowing down leaf spot spread

M. Grabowski, UMN Extension Educator
First spots of daylily leaf streak can be
found in the garden now. M. Grabowski

In the flower garden and in the vegetable garden, the first leaf spot diseases have appeared. At this stage, garden plants still look good and it is easy to overlook a few small spots on lower leaves.

Where did these spots come from? Many are caused by fungal or bacterial plant pathogens that survive in the soil in last years infected plant debris. Splashing rain or irrigation carries fungal spores or bacteria from the soil onto new leaves. These start new infections that become leaf spots. Each leaf spot eventually produce fungal spores or bacteria that can be spread to neighboring plants or within the plant canopy. 

Severe daylily leaf streak at
the end of the summer.
M. Grabowski, UMN Extension
 Leaf spot pathogens spread in splashing rain or irrigation or by sticking to hands and tools. Some can move on wind or moist air currents. With time and the right weather conditions, a few small leaf spots in June can grow into a devastating disease that causes leaves to turn brown, wilt or fall off.

Although there is no way to cure existing leaf spots, there are several things gardeners can do to slow the spread of the pathogen and reduce the damage caused by the disease.

  • Avoid splashing the leaves with irrigation. Use drip irrigation, soaker hose or direct the garden hose at the soil, not the leaves. If sprinklers are the only option, water deeply but infrequently, at the start of a sunny day.
  • Space plants to allow air movement between plants and through the plant canopy. Stake vining
    Removing diseased leaves to reduce
    severity and spread of leaf streak.
    M. Grabowski, UMN Extension
    plants and thin overgrown perennials.  Leaf spot pathogens need moisture on the foliage to start new infections. Good air movement through the garden helps leaves dry quickly after rain or dew.
  • Mulch the soil with wood chips, straw or other organic material. This will keep moisture in the soil where the roots can use it and reduce humidity in the plant canopy.
  • Scout for leaf spot infections in early summer when disease is just getting started.
  • Pinch off infected leaves and compost them.  Each leaf spot produces hundreds of fungal spores or bacteria that can start new leaf spot infections.
    Soaker hose provides water without
    wetting leaves. M. Grabowski
    Removing early infections can reduce the number of leaf spots in the garden later. Never remove more than 1/3 rd. of a plant’s foliage or you will be doing more damage to the plant than the disease.
  • Tolerate some leaf spots. A few leaf spots will not hurt your plant’s overall health. Use the cultural control practices above to keep leaf spot diseases in check and reduce their overall impact on your garden.

Fungicide sprays are not necessary to protect the health of the plant unless the majority of the leaves are killed or dropped as a result of the disease for several years in a row. These sprays will only be effective if applied before disease becomes severe. Fungicide sprays protect healthy green leaves but will not cure existing leaf spots. Because leaf spot diseases spread throughout the season, fungicide sprays need to be reapplied at regular intervals throughout the season to be effective. As a result they are often impractical for a home garden.

When leaf spot disease has been very severe on a particular plant for several years in a row and cultural control practices have not been effective in improving the health of the plant. Consider replacing the plant with a resistant variety or a plant with similar properties (low growing, yellow flowers) from a different plant family. 

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