Identification of Pseudomonas spp. as the causal organism and of sulfate as a promoting factor of midrib rot in butterhead lettuce grown under protection in Belgium

unknown. Since 1995 glasshouse lettuce (Lactuca sativa L. var. capitata) in the province West-Vlaanderen in Belgium regularly becomes damaged by the occurrence of midrib rot. The symptoms occur during the last part of the growing period on the petioles and their prolonging veins (midribs) of the outer wrapper leaves. In most cases the damage is located on the part of the midrib that bends up with an angle of 90ø. Two types of midrib rot can be distinguished. In type 1 the discoloration is blackgreen with a slimy appearance. In type 2 the discoloration rather is light brownish and often is accompanied by light brown spots on both sides of the midrib. Overhead treatment with the antibiotic streptomycin could suppress every symptom of midrib rot. Also, the symptoms could be reproduced on the leaves of healthy plants by injection of the sap of diseased plants in the midrib. These two facts indicate the pathogenic character of the damage. Diagnostic research revealed the presence of Pseudomonas spp. in every case of midrib rot. Overmore, the two types of the disease could be attributed each to a singular Pseudomonas species: type 1 was associated with Pseudomonas cichorii; type 2 with Pseudomonas marginalis or Pseudomonas fluorescens. Development of the very symptoms by injection of suspensions of these bacteria proved they were the primary cause of the disease. Three or four turns of overhead treatment with a solution of potassiumsulfate (25 g per square m of commercial product with 48/ K2O) caused a very significant increase in the number of attacked plants in a holding showing up type 2-symptoms. In another holding, showing up type 1-symptoms, supply of potassiumsulfate did not aggravate the occurrence of real midrib-damage, but only (significantly) that of brown spots in the adjacent area. As the induction of midrib rot was not obtained with overhead treatment of potassium chloride supplying the same amount of potassium as in potassiumsulfate, the deleterious effect should be explained by the presence of sulfate. In the holding with type 2-symptoms, overhead treatment of calciumchloride (44 ml per square m of commercial product of 33/) had a slight (but significant) beneficial effect.