Effect of irradiance upon the population of Pseudomonas coronafaciens in leaves and symptom expression of halo blight of rye

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Abstract
The toxin-induced chlorosis caused by Pseudomonas coronafaciens is influenced by irradiance. Three levels of irradiance caused differences in symptom expression but did not affect the rate of increase or final population of viable cells of P. coronafaciens in rye leaves. Distinct and faint halo blight symptoms appeared in 3–4 days in full light (1425 μ W cm−2), and 58% shade (598 μ W cm−2) respectively. No symptoms or only faint symptoms appeared after 7 days at 86% shade (202 μ W cm−2). When plants kept in 58 and 86% shade were moved to full light 5 days after inoculation, lesion size and chlorosis increased rapidly during the next 2 days. On the 7th day after inoculation, the size of lesions from the 58 and 86% shade treatments exceeded those in full light by 2.5 and 5 times, respectively. A chlorosis index based on lesion size and chlorophyll loss within lesions also reflected this trend although chlorophyll loss was greater in lesions in full light for 7 days. Conditions of low irradiance such as that caused by overcast weather and (or) a dense leaf canopy followed by bright sunshine can cause greater losses from halo blight than a continuous period of high irradiance. Sympton expression may be masked by low irradiance but increase of inoculum is not impaired. Although increased light enhances chlorosis, toxin diffusion or production may be reduced.