Given a fairly wet spring, it’s not surprising that we have a lot of plant disease issues. No, the weather doesn’t cause disease, but it does affect their ability to thrive. But before I go into more details, let me emphasize that plant diseases are not necessarily fatal. Some are; many are simply unattractive at worst. Perhaps these are similar to the common cold in humans; they’re not much fun, but we can outlive it.
Another point to emphasize is that we cannot cure plant diseases except in those cases where we can remove the infected portion by pruning. Fungicides available to us work generally as protectants and may keep the problem from getting worse. That means that disease management generally must be proactive – you have to think about it before it’s there. So what follows is a brief explanation of how plant diseases work and things we can do to minimize their impact.
We’ll start with the disease triangle or the 3 things that are essential for plant disease – a susceptible host, the presence of an infectious pathogen, and environmental conditions suitable for the pathogen to thrive. These are the three areas where we can concentrate our efforts. Sometimes we can manage the host by selecting something that is not susceptible to a common pathogen or that has resistance or tolerance for the pathogen. Some people give up growing roses because the diseases are so challenging; eliminate roses and you don’t have black spot. In some cases plant breeders have provided us with resistant varieties. The “Knockout” roses tend to have some resistance to diseases. Likewise we often select tomato plants labeled VFN for resistance to verticillium, fusarium, and nematodes. Or with all the mildew resistant crape myrtles, there is little justification for planting any non-resistant crape.
The second requirement for disease is the presence of a pathogen or the inoculum. Typically, these are microscopic organisms that are widespread in the soil or are spread in the air, by water, or by humans and our tools. We do not know how to eradicate these things. Sometimes the best we can do is to reduce their numbers by destroying infected plant materials. We call it sanitation and it sometimes includes raking up infected leaves, pruning out infected stems, or removing plants from the garden at the end of the season.
The final leg of the disease triangle typically gives us many opportunities to be good gardeners – environmental management. In this situation we think not only of the warm, humid climate but of the specific micro-climate where plants grow. Since the most common plant diseases are fungi, it’s useful to remember that those organisms usually grow best in areas that are dark and damp – such as the shaded north side of a building, under a kitchen sink, or in a fruit tree that has not been pruned. Pruning trees and spacing plants is one way that we optimize sunlight for all plant parts and increase air movement within the plant to reduce disease pressure.
An important part of disease management is matching the plant with the site environment. Most fruits and vegetables need to be in full sun all day long. While you can grow them in less than optimal conditions, you may also have more disease problems, especially in damp seasons. In addition to good site-plant matches, we cultivate the soil for aeration/drainage; incorporate organic matter or compost; rotate annual crops; mix different crops to slow the spread of pathogens; and avoid excess fertilizer that may make plants more susceptible to diseases.
At this point organic gardeners may like to remind us that their approach is not about products; it’s about philosophy and strategy. With plant diseases the proactive, preventive strategy is the most effective management regardless of your philosophy. Once we have a plant disease, a fungicide may be appropriate in some cases to reduce disease spread within a plant or to other plants. But we won’t cure anything. Taking advantage of environmental characteristics of the site is one of our most effective tools.
A few other strategies may be useful as part of the preventive approach:
Avoidance – Choose disease free plants. Avoid those with obvious problems and look for healthy roots. Use clean tools. We can spread soil borne diseases in soil on tools and we can spread diseases on pruners as well as on our hands. A water hose can remove potentially infested soil from tools when moving from one bed to another. Clean pruners with one part household bleach in nine parts water. And be wary of what you may introduce when importing soil, compost, or mulch.
Eradicate infested material from your gardens. Always be ready to prune the 3-Ds (anything dead, diseased, or damaged) from your plants or gardens. If plants or plant parts are diseased, they serve as a source for further infestation. Always remove dead plant material from your garden at the end of the season. Till in plant residue.
Protect plants by providing good plant care. Plant them in suitable locations with sufficient sunlight and air movement. In enclosed areas removing lower branches from trees, thinning trees, and removing some plants may enhance overall air movement and make it a healthier environment. Use drip irrigation rather than overhead when possible. We can’t control the rain. But water splashing and moving through the plant may be transporting disease pathogens. Avoid working with wet plants; you’re more likely to pick up pathogens on your hands. Use mulch to reduce splashing of disease organisms from the soil to the plant.
To some extent the prevalence of plant diseases is a measure of how proactive you have been. We can’t control the rain and humidity. But we can set plants up to dry out by site management, wide plant spacing, and pruning. We can reduce the spread of diseases by use of mulch and thinking about the effects of our activities in the garden. It’s important that we understand that our climate may favor certain problems and take action to make it harder for those problems to thrive. For almost any plant you have, there is probably a documentation somewhere of what problems are likely and strategies to reduce the severity of the problem. Most of the strategies are preventive rather than cures.
For more on plant disease management, see
http://www.ces.ncsu.edu/chatham/ag/homehort/PltDisease.html
E Letters are archived at http://chatham.ces.ncsu.edu/content/WGEindex and at
http://www.ces.ncsu.edu/chatham/ag/homehort/WhatsGardening/WGEindex.html
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Al Cooke
Extension Agent, Horticulture
Chatham County Center
N.C. Cooperative Extension
PO Box 279, Pittsboro, NC 27312
919.542-8202; Fax 919.542.8246
al_cooke@ncsu.edu
http://www.ces.ncsu.edu/chatham/staff/acooke/home.html