OSU Extension Service Crop Science Report Physiological Leaf Spot and Chloride

Physiologic leaf spot (PLS) is a malady of winter wheat that causes flag leaf spotting and destruction of lower plant leaves in severe cases. Yield losses from zero (when incidence is low) to as much as 10 percent (in severe cases; Smiley et al, 1993a) have been re-ported. PLS leaf lesions (oval to irregular shaped, chlorotic to necrotic spots) are similar to those associ-ated with the fungal diseases tan spot and septoria, but no casual pathogen has ever been successfully isolated from PLS lesions. While foliar fungicides can be ef-fective in treating tan spot and septoria, PLS seems unaffected by such fungicides (Smiley et al., 1993b; Engel et al., 1996). There is a substantial body of evidence that suggests PLS is not a disease, but if not, then what is the cause? Some researchers now believe that PLS is caused by chloride deficiency- that it truly is a physiologic spotting. Chloride has been recognized as a plant nutrient since the mid-1950's (Engel, 1995). In a 1954 paper, Broyer et al described the typical symp-toms of severe chloride deficiency- wilting followed by leaf chlorosis, necrosis and bronzing and branch-ing and clubbing of lateral roots. However, it was believed that chlorine would rarely be deficient in plants "because of its high solubility and availability in soils and because it is also transported in dust or tiny moisture droplets by wind and rain to the leaves where absorption occurs (Salisbury, 1978). " Chloride is generally listed as a micro-nutrient in books such as the Western Fertilizer Guide. It is involved in photo-synthesis and is an essential element in plants for this reason. It has also known to be involved in some plant turgor responses. Chloride has been Russ Karow is an extension agronomist with the Dept