Population fluctuation of Peronospora destructor sporangia in the air and its relationship with the severity of onion downy mildew

In a field trial in the years of 2011, 2012 and 2013, the fluctuation of Peronospora destructorsporangia and its relationship with the severity of downy mildew in onions were quantified. Seedlings of onion cultivar Bola Precoce were transplanted into four replicates of 60 plants. Sporangia were collected by means of a "vane"-type spore collector containing a microscopic slide smeared with vaseline and were weekly quantified by using a microscope. The severity of the disease was analyzed based on the percentage of leaf area affected by the disease. The number of sporangia and the severity of downy mildew in onion were subjected to the calculation of Pearson's correlation coefficient (r). In 2011 and 2012, the collection of sporangia occurred after the observation of the disease probably because the inoculum concentration in the air was low. However, in 2013, sporangium collection occurred before the disease observation but sporadically and at small quantities, suggesting that those sporangia were already present in the air and were captured by the collector. The correlation between the variables sporangium fluctuation and disease severity was significant, r=0.90, r=0.94, and r=0.80 for 2011, 2012 and 2013, respectively. The severity of the disease in onions is influenced by the presence of sporangia collected from the air.