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Powdery Mildew
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Taken from the B.Y.G.L. (Buckeye Yard and Garden Online) Newsletter
Contributing Authors: Pam Bennett, Joe Boggs, Cindy Meyer, Jim Chatfield, Erik Draper, Dave Dyke, Gary Gao, David Goerig, Tim Malinich, Becky McCann, Bridget Meiring, Amy Stone and Curtis Young |
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| There are two very different types of infectious plant diseases known as "mildews", namely powdery mildews and downy mildews. They are very different diseases, caused by different types of organisms, and with different disease cycles, and control strategies. It is important diagnostically to discriminate between powdery mildews and downy mildews since often downy mildews are more serious to plant health and since different pesticides are effective against these very different organisms. With this BYGL item, let's focus on powdery mildews and what green industry practitioners need to know about powdery mildew diseases. First though, to |
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| orient ourselves, here is one perpetual reminder about the nature of infectious plant diseases and their management: the disease triangle. The disease triangle is the simple concept that infectious disease is a process, involving three facets: a virulent pathogen (microscopic organisms such as certain fungi and bacteria), a susceptible host plant, and an environment conducive to disease. A standard test question for pesticide certification always lists these three and asks which of them are necessary for disease to develop. The answer is, of course, all three. Pathogens differ with each disease. If the disease is powdery mildew of lilac, the pathogen is Microsphaera alni. If the disease is powdery mildew of rose there is a different pathogen, Sphaerotheca pannosa. All aspects of the disease triangle are unique for each disease; in the case of rose powdery mildew, development of the disease requires S. pannosa, a rose susceptible to this pathogen, and particular conditions of relative humidity. |
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| Powdery mildews fungi are host-specific. Powdery mildews are some of the most familiar and ubiquitous diseases in the landscape. Familiar examples abound: Powdery mildew of lilac; powdery mildew of rose; powdery mildew of zinnia; powdery mildew of phlox; powdery mildew of dogwood; powdery mildew of ninebark; powdery mildew of sycamore; even powdery mildew of turfgrass. There are over 7000 host plants for powdery mildew diseases worldwide. One of the first things to realize is that these diseases are caused by different powdery mildew fungi. The powdery mildew of rose fungus will not infect lilac.You can cause quite a sensation |
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| by going to a meeting of rosarians and rubbing lilac leaves with powdery mildew on someone's champion mildew-less rose. Though you still may be banned from the meeting, it will not cross-infect. The powdery mildew of zinnia fungus will not infect phlox. The powdery mildew of apple fungus will not infect dogwood. There are exceptions, but these make sense. For example, the fungus that causes powdery mildew of the American planetree or sycamore (Platanus occidentalis) will also infect London planetree. When you realize that London planetrees (P. xacerifolia) are hybrids of the American planetree and the Oriental planetree (P. orientalis), and thus closely related, this cross-infection makes more sense | ||||||||||||||||||
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The Twisted Story of Rose Rosette Disease
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| BYGLers had an interesting discussion regarding the symptomology and treatment for rose rosette disease (RDD) on cultivated roses, particularly on Knockout roses. RDD is caused by a phytoplasm that is spread by the ROSE LEAF CURL MITE (Phyllocoptes fructiplilus), an eriophyid mite that inhabits the shoot tips and leaf petal bases of roses. The mite alone causes little damage; however, the phytoplasm it carries produces a range of symptoms that first become evident in the spring and intensify as the season progresses. | ||||||||||||||||||
| Infected plants produce succulent bright red shoots covered in stunted, twisted stems and leaves. The leaves may also appear red, chlorotic, or a combination of both symptoms and the shoots may be covered by an abnormally high number of thorns. The twisted growth may be mistaken for damage caused by a plant growth-regulator herbicide such as 2,4-D. Currently, the disease is diagnosed based on observed symptoms since there is no laboratory method to detect the phytoplasm. RDD was first reported in 1941 on multiflora rose Manitoba, Canada, Wyoming, northeastern California, and Nebraska. The disease is lethal to multiflora rose and it was originally thought that the pathogen was specific to this non-native noxious weed. However, it has become clear in the intervening years that RDD also infects virtually all cultivated roses. Once plants become infected, all parts of the plants are infectious. Pruners used on infected plants can spread the pathogen to non-infected plants. There are no pesticides available that will control the disease, so management focuses on removing the pathogen by removing infected plants. Entire plants, including the roots, should be removed and destroyed; while the pathogen does not survive in the soil, it will survive in roots. If possible, rose growers should focus their attention on eliminating nearby multiflora rose plants since the plants can serve as reservoirs for this disease. |
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Predicting Blossom-End Rot
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While discussing the impact of the crazy up and down weather this year, the subject of blossom-end rot arose as part of the state and fate of tomatoes, peppers and eggplants this year. Calcium is required in relatively large concentrations for the normal cellular development and growth of a plant. Blossom-end rot is a physiological disorder caused by a deficiency of calcium in the cell walls of fruit. When a rapidly expanding fruit is deficient in calcium, the tissues collapse and begin to break down, resulting in a characteristic dry, sunken lesion at the blossom end of the fruit. Calcium is transported to the fruit and throughout the plant via the xylem as a dissolved nutrient in the water stream. Therefore, anything which affects the uptake and movement of water, with the associated dissolved nutrients, can impact the development of fruit on |
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| tomatoes, peppers and eggplants. As the "Vegetable Visionary," Erik Draper predicted a worse than normal year for blossom-end rot. The predicted prognostication was due to the amount of compromised root systems that he has observed in northeast Ohio. Extreme rainfall amounts led to non-functional root systems. They weren't needed or developed because of the constant rainfall and high humidity during the Spring. Extremely wet soils created ideal conditions for many of the root rotting fungi to invade the non-functional root systems. Some vegetable transplants were minimally watered to try and hold them back and keep them from getting too large before they could be planted out into the fields. Other transplants were over-watered, as people tried to get them to grow fast so as to be first on the market with local produce during the cool, wet Spring weather. Regardless of why it was done, the end result is the same, compromised root systems. Compromised root systems will not move nutrients effectively, so ultimately there are nutrient deficiencies in the plant. Add to those deficiencies, the tremendous physiological stress and cost for a plant to ripen fruit and Erik believes that blossom-end rot will reign. We'll see what the future holds… bacon, tomato and lettuce sandwiches or just bacon and lettuce? |
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