The genus Alternaria consists of approximately 250 species and has a worldwide distribution. As plant pathogens Alternaria spp. infect a diversity of plants. Several different cultivated crops are affected, causing severe damage and crop losses. Alternaria leaf blotch and associated premature leaf drop is considered one of the major problems on apple in many parts of the world. Premature leaf drop in the Golden Delicious apple variety is a global issue that received a great deal of attention in the Netherlands in the 1960s and 70s. While the damage caused by leaf drop is manageable for apple growers farmers, in tree nurseries that cultivate trees the problem can cause a considerable loss of quality of the produced trees. Recent studies performed by Applied Plant Research showed that (specific) sprayings against the pathogen works very well against premature leaf drop. Dead flower bud are a common phenomenon in pear culture in the Netherlands, Belgium and Mediterranean countries, and is also caused by Alternaria spp. The disease is characterized by a partial or complete necrosis of flower buds during tree dormancy. This disease progresses during winter and spring. Eventually, this results in the death of most flowers and decay of buds at flowering. In field trials it was proven that fungicide treatments can reduce disease incidence significantly. There is no information on the identity of the specific pathogens, and whether one or more Alternaria species cause both diseases in the Netherlands. DNA-sequencing and morphological studies were performed on a number of Alternaria isolates from affected apple leaves and diseased flower buds of pear, and several Alternaria groups were identified.
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