The rainy season is off to a strong start this year, with most areas across the state reporting above average rainfall. While rain is generally a good thing in the garden, too much rain can pose challenges, particularly when it comes to plant disease. High relative humidity and excess moisture on plant surfaces are important factors in the development of most diseases, caused by bacterial and fungal organisms.

Most fungal spores and bacterial cells germinate or multiply when water is present on leaves and other plant surfaces. High soil-moisture levels can also promote the development of water molds and root-rotting organisms. If you live in an area that receives a lot of rain, there are several steps you can take to prevent and control bacterial and fungal diseases in your garden.

Most plant diseases are extremely difficult to control once they have become established, therefore it is important to focus on preventative measures.

The first step in disease prevention should be the selection of genetically disease-resistant cultivars, whenever possible. Certain cultivars are naturally disease-resistant, while others have been produced by plant-breeding programs. Plant breeders and researchers at the University of Hawaii College of Tropical Agriculture have spent considerable effort developing fruits and vegetables that are resistant to common plant diseases.

Disease-resistant seeds for the home garden can be purchased online from the UH Agricultural Diagnostic Service Center at www.ctahr.hawaii.edu/seed, or check with local nurseries.

It is important to monitor your plants. Mold, cankers, spots or rot on leaves, stems, roots or fruits; as well as wilting, stunting or rapid death of plants are all signs of bacterial or fungal plant diseases.

Infected plants can rapidly spread disease throughout your garden.

Fungi, for example, can produce thousands of spores, which can be dispersed long distances by wind, or carried short distances by splashing water droplets.

Diseases can spread easily when plants are wet, so it’s best to wait until plants have dried before pruning or handling them.

When pruning or removing diseased plants it is critical to sterilize gloves and equipment before moving from plant to plant to avoid spreading the disease. Most pathogens can survive on dead plant material.

Dispose of diseased plants by burying them deep in the ground, burning them in permitted areas, or bagging and discarding at the landfill. Do not add infected plants to your compost pile.

While you cannot control the rain, to avoid excess moisture, water only when it’s needed and avoid wetting your plants by delivering water directly to the soil.

Widely spacing, selectively pruning and staking plants can also greatly help to improve the aeration around plant foliage and reduce the time that plants remain wet after rains or dewy mornings. Thin out dense foliage to promote good ventilation. Leaves that touch the soil should also be removed.

Improve drainage by using raised beds, containers or mounds. Adding compost and organic matter to your soil can also help to improve soil drainage, especially in heavy clay soils that easily become waterlogged.

Mulching is another effective strategy to help prevent the spread of soil-borne diseases. The mulch layer acts as a barrier to prevent pathogens from splashing onto plants from the soil surface.

Mulching also helps to control other pests, conserve moisture and prevent weeds, another source of plant pathogens. Eradicating weeds will also help increase air circulation in your garden.

For more tips on disease prevention or rainy season gardening, contact your local county extension office. Visit www.ctahr.hawaii.edu/site/map.aspx, call the Master Gardener Helpline 453-6055 or email OahuMg@ctahr.hawaii.edu.

Kim Perry is an extension agent with the University of Hawaii College of Tropical Agriculture and Human Resources and the UH Master Gardener coordinator on Kauai. Email her at kperry2@hawaii.edu.