Michelle Grabowski, University of Minnesota Extension Educator
Regardless of the time and effort put into a garden, inevitably at some point a plant disease will show up. These spots, rots and wilts can be quite distressing to a gardener working to beautify their yard or hoping for a fresh crop of fruits or vegetables. Often the first response to disease problems is to spray a fungicide. Many gardeners however are looking for alternatives to this management strategy. Whether you are going organic, trying to reduce the number of pesticides used in your yard or looking for a simple inexpensive means to reduce plant diseases, cultural control practices can be a great strategy for keeping plants healthy. The US Department of Agriculture Organic rule states that preventive and cultural control practices must be a grower's first choice for pest control. In yards and gardens, preventative and cultural control practices are often effective at reducing disease problems to a level where they are no longer a concern or eliminating them altogether. Combining several of the cultural control practices below can keep gardens healthy without the use of pesticides.
Disease Resistant Plants - Some plants are bred to be resistant to a specific disease. Whenever possible select varieties that have resistance to common diseases. Look for varieties that advertise resistance to specific disease problems like powdery mildew resistant pumpkins or apple scab resistant crab apple trees. General statements like 'good disease resistance' often imply a hardier plant, but these varieties may not include resistance to any specific disease problems.
Scouting and Diagnosis - Examine plants regularly throughout the growing season to find pest problems while they are still minor. Identify the pest causing the problem before taking action. Visit www.extension.umn.edu/gardeninfo/diagnostics for help in identifying unknown pest problems. Knowing the identity of the pest will allow you to choose management practices effective against that particular pest.
Sanitation - If a plant disease problem is identified on a few leaves, stems or fruit, these plant parts should be promptly removed from the garden. Fungal and bacterial plant pathogens reproduce on infected plant parts. Removing infected plant tissue will reduce the growth and spread of the pathogen within the garden. Remember, never remove more than 1/3rd of a plants leaves. In some cases it is worthwhile to completely remove one severely infected plant to prevent spread of the disease to its healthy neighbors. Infected plant tissue can also be removed from the garden at the end of the growing season to reduce the pathogens ability to survive from one season to the next.
Infected plant parts can be composted if the compost pile heats up to 160F. Otherwise infected plant parts can be buried, burned or disposed of in the trash. Follow local city or county regulations regarding disposal of plant material. Many cities offer municipal composting sites for yard materials.
Use drip irrigation or a soaker hose to water plants. This puts water in the soil, where roots can take it up, not on the leaves, where fungi and bacteria thrive. If using sprinkler irrigation, water early in the morning so leaves dry quickly in the sun. Avoid watering as the sun goes down. Wet leaves will remain wet for many hours in the night, providing excellent growing conditions for fungal and bacterial plant pathogens.
Water deeply and infrequently. This will encourage growth of deep plant roots and will allow soil to dry slightly between watering. Continuously wet soil favors the growth of some root rotting pathogens and can suffocate roots. Amend heavy soils with organic matter to improve soil drainage.
Mulch the soil with an organic mulch like wood chips or straw. This will keep moisture in the soil and reduce humidity in the plant canopy. Mulch also helps to reduce the spread of plant pathogens that splash from leaf debris in the soil onto the lower leaves of plants.
Space plants to allow good air movement through the garden. This will help leaves dry out quickly after rain and irrigation.
Tolerate a Non-threatening Disease - Remember not all plant diseases are deadly. In fact many common diseases in the yard and garden affect the aesthetics of the plant more than the health of the plant. Learn more about the plant disease you have encountered at www.extension.umn.edu/gardeninfo before deciding what level of disease control is necessary. Some diseases like oak wilt or Dutch elm disease require action. Others, like powdery mildew on lilac can be tolerated as they will cause no significant damage to the plant.