Physiologic leaf spot and the link to plant Cl nutrition

Chloride has a profound affect on physiologic leaf spot severity in susceptible cultivars. Investigations have revealed the relationship between leaf spot severity (portion of leaf area affected by chlorotic + necrotic lesions) is related to plant Cl according to an exponential-type function. The relationships expressed below are typically of susceptible wheat cultivars. Physiologic leaf spot in Redwin winter wheat and most other cultivars are minor when whole plant chloride concentrations are above 0.10% or 1000 ppm threshold level. As plant chloride drops below this level leaf spot severity increases dramatically.

The threshold level below which leaf spotting is observed may vary slightly with cultivars. CDC Kestrel is considerable more leaf spot susceptible than Redwin. Research suggests its threshold level is approximately 0.15% or 1500 ppm, somewhat higher than for Redwin and other susceptible cultivars.

Figure 1.  Flag leaf spot severity vs. whole plant Cl relationships for Redwin (compilation of 10 field experiments) and CDC Kestrel winter wheat (compilation of 6 field experiments).

The term "Cl-deficient leaf spot syndrome" has been proposed to describe this physiologic leaf spot phenomenon in wheat. This proposal is based on the following considerations: i) the leaf spot problem was associated with soils testing < 1 ppm in Cl; ii) Cl fertilizer prevented or eliminated leaf spot symptoms in cultivars which are susceptible to this phenomenon; and iii) the strong relationship between whole plant Cl and leaf spot severity (portion of leaves affected by chlorotic and necrotic lesions). New research conducted under controlled hydroponic conditions further validates this proposal.