“Polka dot disease” is a common term gardeners use to describe plant issues characterized by spots, often caused by fungal pathogens. While multiple fungi can lead to spotted foliage, anthracnose, primarily caused by Colletotrichum species, is a prevalent example. Understanding these spot-related diseases is helpful for effective management in gardens and landscapes.
Understanding Polka Dot Disease
Anthracnose is a group of fungal diseases, primarily caused by Colletotrichum species. These fungi infect a wide range of plants, including trees, shrubs, vegetables, and fruits.
The disease spreads through microscopic spores dispersed by splashing rain, overhead irrigation, and wind. Contaminated tools and infected plant debris also contribute to its spread. Fungi can survive over winter in fallen leaves, twigs, and contaminated seeds, providing a source of infection for the next season.
Environmental conditions significantly influence anthracnose development. High humidity, cool to moderate temperatures (50 to 68 degrees Fahrenheit), and prolonged wetness on plant surfaces create ideal conditions for spore germination and infection. Densely planted areas with poor air circulation further contribute to these moist conditions. The disease thrives when moisture is abundant, but its spread typically slows during hot, dry weather.
Identifying Polka Dot Disease
The visual symptoms of anthracnose vary depending on the specific plant and fungal species involved. Generally, the disease manifests as dark, sunken lesions or spots on leaves, stems, flowers, and fruits. These spots often start small and circular, with distinct margins, ranging in color from beige, tan, or brown to black.
As the disease progresses, individual spots can merge, forming larger, irregular blotches. Infected leaves may become distorted, curled, or cupped, and in severe cases, they can yellow, brown, and drop prematurely. On trees, defoliation often occurs from the bottom of the canopy upwards. Twig dieback and sunken cankers on stems and branches can also occur with severe infections.
Many common plants are susceptible to anthracnose. Shade trees like dogwood, sycamore, maple, and oak are frequently affected, especially on newly emerging leaves in spring. Vegetables such as beans, peppers, tomatoes, and cucurbits (like cucumbers and squash) are also common hosts. Fruit trees, including mangoes, avocados, and strawberries, can experience significant damage, particularly fruit rot.
Preventing Polka Dot Disease
Proactive measures prevent the onset and spread of anthracnose. Selecting plant varieties resistant or tolerant to anthracnose significantly reduces infection risk. Local nurseries or agricultural extension services often provide information on suitable resistant cultivars for specific regions.
Proper plant spacing promotes good air circulation around plants. Adequate air movement helps foliage dry more quickly after rain or irrigation, reducing the prolonged leaf wetness that favors fungal growth. Garden sanitation, such as promptly removing and disposing of infected plant debris, including fallen leaves and twigs, helps eliminate overwintering fungal spores.
Watering practices influence disease development. Avoid overhead irrigation, which wets the foliage; instead, use drip irrigation or soaker hoses to direct water to the plant’s roots. If overhead watering is necessary, do so early in the day to allow leaves to dry completely before nightfall. Proper pruning, such as removing dead or diseased branches, also helps improve air circulation within the plant canopy and remove potential sources of inoculum.
Managing Polka Dot Disease
Once an anthracnose infection is present, continued cultural practices are important for management. Immediately removing and destroying severely infected plant parts, including leaves, twigs, and fruit, reduces fungal inoculum. Sterilizing pruning tools after use prevents accidental spread to healthy plants. Improving soil drainage and applying mulch also helps by reducing soil splash onto lower leaves, which can carry spores.
For severe or recurring infections, fungicides can be considered, but they are generally more effective as protectants rather than cures for established infections. Fungicides work by protecting healthy plant tissue from becoming infected. Application should typically occur early in the season, often before buds open or when new growth emerges, especially if cool, wet conditions are expected.
Copper-based or chlorothalonil-based fungicides are often mentioned for anthracnose control. However, for many common landscape trees, chemical treatments may not be necessary as trees often recover and produce new leaves after initial defoliation.