How to Identify Common Plant Diseases

Mold in the garden is a common sight, especially during extended bouts of rainy, cool weather or high humidity. High moisture levels help mold to grow and spread. In botanical circles, mold that forms on dying flowers or leaves is known as botrytis or gray mold. Botrytis spores are present in soil and frequently already on plants that you buy. It lies dormant until conditions are right for it to blossom and spread. The best prevention is to remove dead or dying leaves, flowers and fruit, and to mulch soil so that rain can’t splash spores onto plants. Flowers that often get botrytis include zinnia, peony, geranium — plants with blossoms that are full and packed with petals.

Warm air temperature and days of rain can lead to leaf spot diseases on many different plants, including flowers, vegetables, trees, shrubs and herbs, like this golden oregano. Leaf spots originate with bacteria or fungus, both of which reside in soil or on nearby plants. The combination of warm air and rain foster ideal conditions for an outbreak, especially when leaves stay wet 24/7. Leaves develop individual spots, which eventually coalesce and kill the leaf. Once a leaf falls, filled with infection, it slowly rots and infects the rest of the plant. Gather and destroy any leaves that fall as a result of leaf spot diseases. When rain stops and plants dry out, they will usually outgrow the symptoms. This golden oregano looked beautiful about two weeks after this outbreak and stayed healthy the rest of the season. To prevent leaf spot, make sure mulch covers soil beneath plants to keep disease from splashing onto leaves.

Powdery mildew is caused by a fungus that multiplies when soil is dry and humidity is high. It often appears on plants in late summer. The first easily visible signs on leaves are white spots of fungus. Over time, the fungus grows and forms a network on the leaf, connecting the spots. After the fungus overtakes a leaf, the infected parts turn brown and disintegrate. Plants typically attacked by powdery mildew include bee balm (shown), zinnia, roses, lilac, squash, beans, cucumbers and tomatoes.

This is what powdery mildew looks like when a fungus-ridden leaf starts to fall apart. There are many different types of powdery mildew; they’re actually rather specific to the plants they infect. The fungus that attacks your squash won’t, for instance, spread to your lilac. Treatments include disposing of infected plant material and cleaning up the garden well in fall to remove any infected leaf or stem residue. Destroy infected plant parts; do not add to your compost pile. Mulching beneath plants helps preserve soil moisture and prevent fungus from splashing from soil to leaves. It also helps if plants have good airflow around them — space them properly.