Plant disease management strategies

By Al Cooke
Posted Thursday, July 10, 2008

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Pittsboro, NC - This spring and early summer have been good for some plant disease problems. In some cases, such as fire blight of apple and pear or dieback in azaleas or rhododendrons, we can remove diseased plant parts with pruners and arrest the problem right there. But with problems such as cedar apple rust on apples or powdery mildew on squashes and melons (or roses, crape myrtles, verbena, dogwood, and a plethora of other plants) pruning may be neither practical nor effective. Since we can’t cure plant diseases, perhaps it’s a good time to review some of our best disease management strategies.

Many plant diseases thrive in an environment that is damp and dark when the temperature is suitable for growth. We are currently in what I think of as a normal summer weather pattern of evening thunderstorms. While none of us is complaining about rain, this situation leaves plants wet all night in high humidity (damp and dark!) with temperatures in the 70s. When we go out in the morning and experience the damp and muggy air as a bit uncomfortable, plant diseases are thriving.

We can’t cure these things. Even if you’re willing to use anything in the handbook of agricultural chemicals, there is nothing out there that you can put on your squash or roses to cure powdery mildew. So prevention is our primary strategy. Preventive strategies focus on managing the plant environment to enhance plant growth while making it more difficult for pathogens. So what can we do to make the plant environment drier? Think sunshine and air movement.

Sunshine and air movement are why I begin classes by discussing site selection. I find many people either overestimate the amount of sun they are getting or underestimate its importance. Sunlight is not only the driving force in producing all the flavors, textures, colors, and nutrition in the foods we eat. It also helps to dry things up. If your garden doesn’t get sunlight until 10 or 11 in the morning, you have left extra hours for pathogen growth at the expense of production.

The ideal spot for your garden or orchard gets full sun all day long. If you can’t provide that, then you have to accept that some crops will be less productive. The seriousness of that issue is varies from year to year. Last year we had fewer problems with humidity and fewer disease problems. This year we had a damp spring and recently the wet evenings. Some years you’ll be more successful than others.

Another way to dry things out is to have good air movement. While golf courses may get a bad rap for “all those chemicals,” they are also among the few places where I have seen fans used for managing the plant environment. You may not find it practical to place fans in your garden, but that does take me back to site selection. Open space where the wind blows is optimal. If you’re in the woods, you’re going to be seriously limited. Removing dense vegetation or lower limbs of trees may enhance air movement.

If your site is not ideal, then you may want to rethink what to grow. If you’re dealing with powdery mildew and can’t provide full sunlight all day and air movement, then you may reconsider growing any of the plants mentioned above. Most plants have some potential for disease problems; some are known for their problems. When people don’t know what is wrong with their squash or cantaloupe plants, I have a litany of possibilities and symptoms that we can discuss. But whatever the diagnosis, managing the plant environment will be a primary management strategy.

Soil preparation is something most gardeners do better for vegetables than for any other type of plant. Perhaps that is why we see more instances of root pathogens with ornamental plantings. With poor soil drainage (damp and dark) we have an enhanced opportunity for the soil borne pathogens to flourish. When roots rot, then the plant doesn’t get the water, oxygen, and nutrients it needs to thrive. Root disease symptoms may resemble drought or nutrient deficiency.

Crop rotation while not available in permanent plantings is the oldest and perhaps most reliable strategy for managing some plant diseases. While a soil borne pathogen may reduce a plant’s productivity, it may not even be noticeable in the first year. But with a suitable plant host, the pathogen has opportunity to grow and reproduce. With a suitable plant host in a second or third season, the pathogen gets a competitive advantage. An area where you didn’t see a problem last year may be an area where you cannot grow that crop this year. Even if you have only a 4-by-4 plot, sketch it out in four quadrants and keep a record of what is grown and where for each year. Allow at least 3 years between growing things from the same plant family.

Plant spacing is part of managing air movement. You can often grow more plants in a given area by planting them closer together. But you don’t necessarily get more productivity. Productivity is at least partly related to how many leaves are effectively doing photosynthesis. If leaves are crowded so that exposure to the sun is limited, the individual plant is less productive. If leaves are affected by diseases, they are no longer functional. Whether it’s number of inches between spinach plants or number of feet between fruit trees, optimal spacing (and air movement) is part of disease management.

With fruit trees, as well as other trees and shrubs, training and pruning are important management strategies that enhance light penetration and air movement through the plant. During the summer when the fruit tree is completely loaded with leaves and fruit, it is a good time to see how much sun is reaching the ground below. A ballpark target is that in the middle of the day about 10% of the ground below the tree should be spotted with sunlight. If you’re not getting that, more pruning may be in order.

Training trees in their early years is a good way to enhance air movement through the tree. While many people shy away from doing anything to interfere with the tree’s growth, selecting which branches to keep and which to remove allows you the opportunity to determine branch spacing within the tree and to enhance air movement (and drying) through the plant canopy. All too often the questions I get are how to prune a tree that is 5-10 years old and has never been pruned. At that point I do not have any good guidelines. In many cases it would be more productive to remove the tree and start over.

If I could sum this all up, I would suggest that disease management is not primarily about what you do after you have noticed a disease problem. Disease management is about what you do proactively to prevent disease from getting established and minimizing its impact once it’s there. For almost any plant you want to grow, there is a wealth of information about what diseases or insects you might encounter. Now is a good time to learn more about those problems as you plan for next year.