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Remarks (internal):May be distinguished from V. albo-atrum (CMI Descript. 255) by the presence of true microsclerotia forming colonies which are entirely black in reverse with growth at 30°C (Isaac, 1949; Heale & Isaac, 1965; Skatow. 1969). On plant tissue the basal parts of the conidiophores in V. albo-atrum are often brownish (Smith, 1965). The var. longisporum Stark, with conidia often almost twice as long as V. dahliae and known only from Armoracia rusticana, appears to be a stable diploid (fide Ingram, Trans. Br. mycol. Soc. 51: 339-341, 1968). Disease control depends mainly on the use of soil fumigation and development of resistant varieties. Crop-rotation measures have generally been less effective than with diseases caused by V. albo-atrum. Development of systemic fungicides offers further means of control in future.
Description type:Non-original description 
Description:Verticillium dahliae Klebahn, Mycol. Centralb. 3: 66, 1913.
= Verticillium dahliae var. longisporum C. Stark, 1961.
= Verticillium albo-atrum var. medium Wollenw., 1929.
= Verticillium albo-atrum auct. pro parte.
Cultures growing rapitly on potato-dextrose agar and malt agar at 23°C, the prostrate hyphae first produced hyaline. Mycelium becoming flocculose and white, rather more densely compacted on PDA than MA, hyaline, whitish to cream in reverse after 1 week, later becoming black with the formation of microsclerotia; hyaline sectors arising very frequenty in the generally white colonies. Conidiophores abuntant, more or less erect, hyaline, verticillately branched, 3-4 phialides arising at each note, phialides sometimes secondarily branches. Phialides variable in size, mainly 16-35 x 1-2,5 µm. Conidia arising singly at the apices of the phialides, ellipsoidal to irregularly sub-cylindrical, hyaline, 0 (-1)-septate, 2,5-8 x 1,5-3 µm in var. dahliae; 5-12,5 x 1,5-3,5 µm in var. longisporum. Resting mycelium dark brown, only formed in association with microsclerotia. Chlamydospores absent. Microsclerotia arising centrally in cultures from a single hypha by repeated budding, dark brown to black, torulose or botryoital, consisting of swollen almost globular cells, very variable in shape, elongate to irregularly spherical, very variable in size, 15-50 (-100) µm diam.
Hosts: On a wide range of herbaceous and woody dicotyledonous plants. Engelhart (1957) gives a host index for V. albo-atrum which includes hosts of V. dahliae, but many reports attributing disease to V. albo-atrum almost certainly refer to V. dahliae. Moore (1959) lists the British records. Economically significant diseases are caused in cotton, strawberry, stone fruits and various solanaceous crops.
Diseases: Causes 'wilt' diseases: diurnal flaccidity followed by permanent wilting often indicate the onset of these diseases, but do not always occur. In herbaceous plants the symptoms normally affect the leaves in acropetai succession (but basipetal in mint), often showing one-sided pattems of chlorosis and necrosis that result from infection of only a few vascular bundles. Infected xylem commonly shows brown discoloration. Heavy infection in some hosts (e.g. tobacco) causes foliar chlorosis with progressive marginal and interveinal necrosis and defoliation. In trees and shrubs, individually affected branches show either rapid collapse and death of leaves or gradual yellowing and premature defoliation. For detailed discussion of symptom patterns in vascular diseases see Talboys (1968).
Geographical distribution: Widesptead in temperate and sub-tropical regions (see CMI Map 366, ed. 2, 1969).
Physiological specialization: Although isolates from many hosts can cause disease in a range of other species and genera and can infect still more without causing obvious symptoms, some host-specificity has been demonstrated in V. dahliae from Brussels sprouts (36: 743), peppermint (Nelson, 1950) and pepper (38: 378).
Transmission: The pathogen persists in the soil for long periods, initially in debris of infected host-plants, possibly later as free microsclerotia, infecting roots after contact. Disease problems commonly arise from repeated cropping with susceptible species, e.g. potato, tomato, strawberry. Activity of certain tylenchid nematodes may predispose some hosts to infection by Verticillium (40: 261). Spread of disease can result from dissemination of debris from infected plants and the use of planting stock vegetatively propagated on infested land. Transmission by superficial infection of seed has been reported for cotton (30: 367) and sunflower (39: 417).
Literature: Isaac, Trans. Br. mycol. Soc. 32: 137-157, 1949; Isaac, Trans. Br. mycol. Soc. 36: 180-195, 1953; Smith, N.Z. Jl agric. Res. 8: 450-478, 1965; Stark, Gartenbauwissenschaft 26(8): 493-528, 1961; Heale & Isaac, Trans.Br. mycol. Soc. 48: 39-50, 1965; Skadow, Arch. PflSchutz 5: 155-166, 1969 (taxonomy); Engelhard, Host index ofVerticillium albo-atrum Reinke & Berth. (including Verticillium dahliae Kleb.), Pl. Dis. Reptr Suppl. 244, 1957;Moore, British parasitic fungi, 1959 (hosts): Talboys, Ch. 6 in, Water deficits and plants growth, vol. 2 (ed. T.T.Kozlowski), 1968 (symptom patterns in vascular diseases); Nelson, Tech. Bull. Mich. Sta. Univ. agric. Exp. Stn221, 1950; Blodgett, Verticillium wilt of stone fruit in Washington, Circ. Washinrton agric. Exp. Stn 425, 1963;Parker, Verticillium hadromycosis of deciduous tree fruits, Pl. Dis. Reptr Suppl. 255, 1963 (disease).Where the text of a paper refers to the presence of microsclerotia it has been assumed that the organism concernedis V. dahliae even though it may have been assigned by the author to V. albo-atrum.

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