Leaf-scar infection in relation to the olive-knot disease

Abstract does not appear. First page follows.

Introduction

Olive knot, a serious disease of Oleo europea L. in most of the olive-producing districts north of the Tehachapi mountains of California, is characterized by the development of overgrowths, knots on the branches. These knots develop most frequently at the leaf scars except in years when freezing injury occurs. The present study furnishes inoculation data, with histological evidence that the scars, under certain conditions, are portals of entry of the causal agent, Bacterium savastanoi E.F.S., into the host. This paper also deals with inoculation experiments used to determine the length of time during which the leaf scars are susceptible to infection. It further describes the microchemical and histological studies of the development of the abscission region just before and after leaf fall, and thereby elucidates the role played by leaf scars in infection.

Prevalence of Leaf-Scar Infection

Observations in California olive groves show that a large percentage of the new knots forming each year develop at leaf scars. Horne, Parker, and Daines ⁽²⁾4 were first to point out this fact: “By far the largest number of knots appear on leaf scars or wound callus.” Wilson ⁽⁶⁾ also recognizes the importance of leaf scars as infection courts. He points out that in years other than those in which freezing injury occurs, as high as 90 per cent of the new knots on branches develop at leaf scars.

The distribution of inoculum from active knots necessary to infect leaf scars depends upon the presence of dripping moisture, as from rain. Horne, Parker, and Daines ⁽²⁾ state that the bacteria exude in a slimy mass from the fissures of knots during rain. Wilson ⁽⁶⁾, in further and more extensive studies, demonstrates the importance of rain in the exudation and spread of the organism as related to infection. He shows that enough bacteria were present to cause infection within a short period after the knots were moistened.