120
D ISEASES
OF
V EGETABLE C ROPS
Phloeospora herclei
PHLOEOSPORA LEAF SPOT
101
Introduction and significance Phloeospora leaf spot is generally a minor leaf disease of parsnip. In the UK severe infection usually occurs on late summer and autumn parsnip crops. A Phloeospora disease also has been reported from New Zealand. Symptoms and diagnostic features The first symptoms are small (1–2 mm) pale green or brown leaf spots. As disease develops, leaf spots increase in number, spots coalesce to form large necrotic blotches (2–5 cm), and infected leaf tissue becomes gray brown (101). The disease spreads rapidly through the crop, causing extensive leaf death and defoliation (102). No root symptoms have been reported. Severely affected leaf tissue has tiny black fruiting bodies called acervuli. Spore tendrils ooze from acervuli and form characteristic white patches on leaf surfaces (103). Causal agent Phloeospora leaf spot is caused by the fungus Phloeospora herclei. Fruiting bodies are minute, cupshaped structures called acervuli. Conidia are formed in these structures and are curved, one to four septate (though most are single septate), and measure 50–80 x 3.5–5 μm. The early literature probably confused P. herclei with Ramularia pastinaceae. The Phloeospora leaf spot disease reported from New Zealand is attributed to P. crescentium. However, P. herclei and P. crescentium may be the same organism.
F UNGAL D ISEASES
Disease cycle Little is known about the epidemiology of this pathogen. Inoculum is spread by splashing water and direct contact between leaves. Phloeospora herclei probably survives on infected debris and on weed hosts such as hogweed (Heracleum sphondylium).
101 Necrotic leaf blotches of Phloeospora leaf spot of parsnip.
102
102 General necrosis of parsnip foliage caused by Phloeospora leaf spot.
103
Control Specific control measures have not been developed. References Laundon, G. F. 1970. Records of fungal plant diseases in New Zealand. New Zealand Journal of Botany 8:51–66. Riley, E. A. 1952. Leaf spot of parsnip caused by Phloeospora crescentium (Barth.) n. comb. Mycologia 44:213–215.
103 White spore masses of Phloeospora leaf spot of parsnip.