Disease Prevention in the Garden Begins Now
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Experienced gardeners are well-aware that we face many disease management challenges in vegetable gardens in our region. While some years are worse than others, our unique climate and topography set us up for regular challenges. Hot days and cool nights, along with humidity, lead to regular dew formation overnight. The periods of leaf wetness help fungal and bacterial diseases proliferate quickly. This effect tends to be worse in mountain valleys, where cool air sinks overnight, thus leading to increased condensation and dew. Regular rainfall events also contribute to leaf wetness and can cause soil to splash onto leaves, which can cause an initial infection by pathogens that survive in the soil. Many of our common vegetables are not native to our area, and therefore are not naturally adapted to our local weather conditions. In a way, we should expect an uphill battle with these diseases. On the other hand, our climate is actually very good for growing many favorite vegetables. With a good plan and diligent management, we can keep diseases at bay, and attain excellent harvests.
Common vegetable diseases can affect foliage, fruit, and roots. The diseases are usually bacterial or fungal. Most of them need to reproduce to infect plants, and many of them do so with periods of wetness. Some are present in our environment all the time, and others travel in on weather systems or on infected plants. For example, septoria leaf spot, a fungal disease of tomatoes, survives in soil and infects foliage when rainfall splashes spores onto leaves. It is always going to be a challenge for gardeners. Early blight is another that affects potatoes and tomatoes- these affect foliage worst and are often simply referred to as blight. They often start in lower leaves or in areas with poor air circulation and spread when temperature and humidity conditions are favorable. They do not kill the plant unless left unmanaged, and do not always affect fruit, but they could affect yield if severe.
In contrast, late blight is a fungal-like disease that does not overwinter in our climate. It travels into the area on weather systems from tropical places where it can overwinter, or on infected plants grown in greenhouses where the pathogen could overwinter. Once it arrives, assuming favorable conditions, it can destroy a plant quickly. It is important to understand the life cycle of the disease to understand how to best control it.
Your best defense against diseases is to use good cultural practices. Provide adequate water and fertilizer, use mulches to, and properly spacing plants. Vegetables need at least 1” water each week- apply what they do not receive with irrigation. Use soaker hoses or drip irrigation systems to reduce water on the foliage. If you do not have access to these systems, water in the early morning, and aim the hose at the ground around the plant base to reduce contact with leaves and fruit. Support vining crops with trellises to keep them off the ground and create good air circulation. Prune out lower and diseased leaves regularly. Mulch the garden with straw mulch, landscape fabric, cardboard, or newspaper to keep reduce soil splash onto leaves, conserve water, and keep weeds down.
Select disease-resistant varieties if you experience any persistent problems, such as powdery mildew. Many species have varieties that have been bred and selected to by reading labels carefully. Contact your County Extension agent with help in diagnosing these diseases. Select transplants that are healthy and free of insects and diseases. Bacterial spot of pepper affects fruit and foliage, and while worse with wet weather, it usually begins because an infected transplant is set out.
Some fungal and bacterial diseases live in the soil and enter through plant roots. The disease may always be present in your soil, but may only become pathogenic, damaging your plant, during periods of high rainfall, when plant roots are damaged from soil saturation. If your soil stays saturated for long periods after rainfall, consider constructing raised beds to promote drainage and reduce saturation. Choosing varieties resistant to soil-borne problems specific to your garden site can sometimes be the only solution to these challenges. For example, tomato varieties labeled as VFN indicate a variety resistant to verticillium wilt, fusarium wilt, and root-knot nematodes. If you have soils that have Fusarium wilt or root-knot nematodes, these resistant varieties or tomatoes grafted onto resistant rootstocks are essential for obtaining good harvests. Bacterial wilt is another soil-borne disease that can only be overcome by planting resistant varieties or plants grafted onto rootstock that has resistance to bacterial wilt.
Stay on top of challenges with a visit to the vegetable garden every day or two. Remove diseased foliage when the problem first appears. Preventive fungicides are often recommended on tomatoes, and sometimes on peppers, eggplant, cucumber, melon, and squash.
Most fungicides are protective or preventive rather than curative. They can help prevent infection or spread of a disease but do not cure the plant, per se. Therefore they must be applied prior to infection events in order to actually control the problem. Often, they need to be sprayed every 7-14 days, according to label directions, to be effective.
For those seeking to reduce spraying the cultural practices mentioned above are imperative. Those wanting to use Organic practices can use fungicides with the active ingredients Bacillus subtilis, Bacillus amyloquofaciens, copper octanoate, sulfur, and a few others. Look for the OMRI (Organic Materials Review Institute) symbol to indicate a product is approved for organic gardening. Other common vegetable fungicides include chlorothalonil and other copper fungicides. With any fungicide, be sure to follow label instructions on application timing in regards to daylight and temperature, rainfall events, and pre-harvest interval. For safety, they may require that you not spray within 1-4 weeks of harvest or longer. Others can cause damage to the plant if sprayed in high temperatures. Pay attention to any personal protective equipment (PPE) requirements indicated in the label. These are in the label to protect you and remember, the label is the law.