| Powdery Mildew: Another Look |
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In the nursery trade more and more crops are being affected by powdery mildew. A three-year research project — undertaken by the Netherlands ‘Productschap Tuinbouw’ in collaboration with the Belgian ILVO Institute, which has expertise in identification and control techniques — was started in 2008 in order to better understand the biology of the different powdery mildew fungi, their potential plant damage and one or more efficient control techniques. On a portion of the crops that make up the nursery trade plant assortment, we see powdery mildew — a hairy growth of hyphae with white spots on the leaf or stem. However there are also plants where powdery mildew is much less visible, yet still causes significant crop damage. To get a better a better handle on the various powdery mildew fungi in nursery crops and the best way to control them, a three year project was started in 2008. In the first year, the emphasis has been on a literature search and new laboratory investigations to determine the sources and development of powdery mildew in several nursery crops. A genus can be affected by more than one species of powdery mildew. A species of mildew can be capable of affecting several genera. And a given plant can be affected by more than one species of powdery mildew, sometimes at the same time. Proper identification of the causal agents is important, as they often react differently to various control techniques. Powdery Mildew SpeciesIn the table below is a grab bag of various powdery mildew species and associated nursery crops. This list is the result of the literature study and the new laboratory investigations during the first year of the study. Noteworthy are the lab diagnoses of powdery mildew on Magnolia stellata and Rhus, which had not been previously observed in Europe.The study highlights the diversity and quantity of powdery mildew fungi that are present in the nursery industry. At the end of the growing season, the powdery mildew fungus develops cleistothecia (fruiting bodies) on the surface of the fungal hyphae. These manifest themselves as small black or brown speckles (0.1 - 0.2mm) with appendages (fulcra) that can be observed under the microscope and from which the exact genus and species of the fungus can be determined. These differences can, in part, determine whether a control practice succeeds or fails which is why it is important to determine the exact species of powdery mildew that one is dealing with. During the past season, cultural advisors have come across powdery mildew on a regular basis. Not only on the usual culprits such as Acer and Rosa, but also plants such as Catalpa, Magnolia and Hydrangea were diagnosed with a lot of powdery mildew, the visible symptoms of which were not always easy to identify. In Rhus and Hydrangea, the symptoms displayed themselves both on the lower and upper leaf surfaces as dark purple-red blotches, with, very occasionally, a tuft of white hyphae. Magnolia leaves often showed a yellow-brownish discoloration, with, very rarely, the characteristic white tufts. These were in contrast to the brownish color of powdery mildew on Rosa and Magnolia. There were also significant variations in the timing of the appearance of mildew. In crops such as Weigela, Corylopsis and Wisteria, infestations were also found the causal agents of which have not yet been determined. They will be the subject of ongoing study in 2009. The ProjectIn the first year of the three year PT-project ‘Effectively controlling powdery mildew’, the emphasis was on mapping the causal agents and their culturally important host plants. The results, coming from a literature study, laboratory tests and practical experience of the advisors involved, are a basis from which the rest of the project will flow. During 2009 and 2010 trials with various crops will determine the importance of the various powdery mildew fungi to the crops in question on the one hand, and effective means of control on the other hand. The stakeholders (i.e. grower groups) will be kept in the loop regarding the findings of the study. They will then be able to direct the follow-up within the project. This will allow the group to address very specific problems as part of the study. Table: Powdery Mildew species and their plant hosts
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