Phytophthora porri Foister, the Causal Fungus of the Leaf Blight and Bulb Rot of Scallion, Allium bakeri Regel

Abstract

The bulb rot and leaf blight of scallion, Allium bakeri Regel, is one of the most serious problems in scallion cultivation in Fukui and Kyoto Prefectures, as it occurs in many places, especially in the season of low temperature, between September and April.
The causal fungus was identified as a soil fungus, Phytophthora porri Foister, which is known as leek fungus in England and Holland. This fungus causes the white tip of the leaf, leaf blight, bulb rot and root rot of scallion in the field and storage.
Bacteria and Fusaria, which had already been reported by some investigators as the causal microorganisms of the bulb rot of scallion in storage, seemed to be of secondary importance in most cases.
In the field, the disease occurs with the appearance of white tip or blight of leaves after the rains in mid-autumn. The symptom is then extended to the bulb and root, resulting in the soft rot of the bulb. Direct infection of bulb and root is also frequently observed in the field, which is directly responsible for the patches often observed in scallion fields.
Sporangia of this fungus are formed in abundance when the diseased leaves are either kept wet by rain or dew or dipped in water. The sporangia are long ellipsoid or obpyriform in shape, and mostly 24.7-57.7×16.6-39.2μ (36.5×25.2μ on the average) in size. Sexual organs are produced abundantly in unpaired cultures on agar media. Antheridia are mostly paragynous and rarely amphigynous. Oospores are spherical, yellowish brown, and 18.0-36.7μ (29.3μ on the average) in size. No chlamydospore is observed. The fungus is capable of growing in the temperature range of 0-27°C, the optimum being in the range of 15-20°C.
The host range of this fungus was found to be as follows: scallion (Allium bakeri Regel), Welsh onion, onion, gynmight (Allium tuberosum Rattler), garlic, and wild rocambole (Allium nipponicum Franch. et Say.).