CULTIVATION AND TECHNOLOGY Secrets revealed about Early blight and Pseudo-early blight In the summer of 2009, the HLB in Wijster carried out a study into the occurrence of Early blight in the Netherlands. The study was performed as part of the 4D-project. The study showed that the foliage lesions that were found on potato crops early in the season of 2009 and resembled Early blight proved not to be due to an Alternaria solani infection, but are connected with Pseudo-early blight, caused by ozone stress and boron deficiency. T he HLB carried out an in-depth study into Early blight in 2009 to discover the cause of foliage lesions in the cultivation of potatoes. The study showed that most of the lesions had nothing whatsoever to do with Early blight. Yet the fact remains that many of the crops died off at an early stage in 2009, following the development of lesions that are seen in Early blight. These are lesions with the typical concentric circles also found in Early blight. The question now is: what could have been the cause of these lesions. At the beginning of November 2009, one of the researchers remembered a case with Early blight type lesions in 2006, which we will call Mimi. The Mimi case On 23 June 2006, a potato field in the Northeast Polder was visited where a number of potato varieties had been planted to test their suitability for cultivation under Dutch weather conditions. One of the varieties in the field was the low-growing Mimi variety. It was reported that this variety was found to be infected with Early blight, which was rather strange, because Early blight had not been reported anywhere in the country. It was indeed true that the foliage showed lesions resembling Early blight: i.e. lesions with clear concentric rings that are usually attributed to Early blight. Leaves with lesions were collected and taken to the HLB for further inspection. Microscopic examination showed that no trace of fungal infection on or in the lesions was found. Tissue tests of 30 lesions in one case provided a somewhat bigger lesion, C. cladosporioides. The other lesions appeared to be fungi free. On 29 June, the field in question was inspected again. The lesions appeared much bigger now, and parts of the foliage were dying off as a result of the concrescent lesions. Again, material from thirty lesions were examined. All lesions now proved to have C. cladosporioides and one big lesion contained A. alternata. Apparently, C. cladosporioides had been able to form a colony of lesions and A. alternata was a bit slower. Because no pathogens could be found in the vast majority of lesions and a highly raised level of ozone was measured, the symptom was identified as leaf speckle or mud mottling, which is a symptom of ozone damage that resembles Early blight. The disease can best be described as Pseudo-early blight. Pseudo-Alternaria. Foliage of the Mimi variety with lesions resembling those of Early blight with concentric rings without the presence of the Alternaria solani pathogen causing the disease. Foliage of the Mimi variety, a week later than in Photo 1. Lesions have concresced and parts of leaves are dying off. Note the concentric rings of the lesions - Pseudo-Alternaria. Potato World 2010 • number 4 21 Pagina 20

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