Studies on white leafspot of turnips caused by Cercosporella brassicae (Fautr. and Roum.), Hoehnel

Abstract

Cercosporella brassicae (Fautr. and Roum.), Hoehnel causes white leafspot of turnips (Brassica rapa. L.). The fungus attacks turnip plants at all stages of growth, and infects a wide range of host plants almost all in the family Cruciferae.

C. brassicae was isolated from infected leaves on PDA, V-8 and Water agar (WA). Isolation was achieved by streaking media with single spores or plating small leaf sections with lesions. The colonies, formed by isolates from the turnip leaves, on PDA, water agar and V-8 were first gray, later turning dark and olivaceous-brown. In WA, the fungus secreted pink pigments which could be seen near the edges of the colonies. Colonies on PDA, V-8 and WA had thick-walled, septate, brown mycelia with stroma-like or sclerotia-like structures. Young hyphae were hyaline and thin-walled.

When C. brassicae was cultured on PDA and V-8 juice agar, growth was relatively faster on PDA. When light was excluded from the cultures, growth was still much slower on V-8 compared with that on V-8 under 12 hours light and dark cycles. Addition of yeast extract had no marked effect on radial growth of the fungus on both media.

Conidial production only occurred on V-8 juice agar, conidia forming soon after colony establishment, starting from the centre of the colony. The zone of production could be seen as a gray area in the centre of the colony. The zone increased in size as the colony extended radially. At two weeks, the zone of conidial production lagged slightly behind the colony edge and then a new zone of fresh conidia started forming in the third week. The conidia were borne singly on the conidiophores. The conidia were 70-100 µm long, septate, hyaline, cylindrical, straight or scolecoid. The conidiophores were short (2-13 µm long) straight or slightly geniculate.

When cool white light with a significant amount of near ultra violet radiation (310-410 mm) was supplied from the sides of the culture plates, conidia production was less and also very much reduced in 20% V-8 juice agar compared to 5% V-8 juice agar. This was partly because of the filtering effect on the light spectra which could not reach the submerged mycelia at the centre of the plates. But when light was supplied from above the culture plates, conidia production was increased, with 20% V-8 juice agar having the highest number of conidia per unit area.

The amount of V-8 juice in the media was important in determining the number of conidia produced, numbers of conidia increasing with increasing concentration of V-8 juice.

Conidia of C. brassicae did not exhibit endogenous dormancy. They were capable of germinating within a few days of their formation. Conidia germinated mostly from the end cells. Germtube formation from middle cells appeared to follow those from end cells. The same mode of germination was exhibited on the leaves. Although conidia did not require exogenous source of carbon to germinate, they still needed it for germtube formation. This was observed when only a few germtubes per conidium were formed on WA compared to PDA and V-8 juice agar.

Self-inhibition was exhibited by conidia of C. brassicae. At a concentration of 1.5 x 106 conidia per ml, germination was very poor, indicating high self-inhibition. This was tested statistically and found to be significant. At 1.2 x 104 conidia/ml all spores germinated.

Germination was inhibited in vitro by captan, iprodione and mancozeb. Germination was not inhibited by prochloraz even by high concentrations (1700 µg/ml).

When the conidia were germinated in vivo, penetration of the stomata by germtubes or primary hyphae appeared random without any oriented movement. In some cases appressoria-like structures formed at the stomatal openings. Germtubes or primary hyphae often bypassed the stomata without penetration. Development of lesions seemed to depend on the temperature during incubation in the glasshouse. When temperature was above optimum (20-24ºC), as happened in the glasshouse, lesion development was delayed.

For control four fungicides were tested: captan, iprodione, mancozeb and prochloraz. When tested as protectants, all were effective except captan which did not provide complete protection. Secondly, two fungicides: iprodione and prochloraz were tested as curative fungicides. Prochloraz had a complete curative effect presumably because of its good translaminar action. Iprodione had some curative action on C. brassicae, but some establishment of the fungus in leaves still occurred.