«Association of Aphanomyces cladogamus with severe root rot of pansies By C h a r l e s D r e c h s l e r (Horticultural Crops Research Branch, ...»
When portions of natural or artificial substratum well permeated with mycelium of the pansy Aphanomyces are shallowly immersed in distilled water the fungus extends numerous hyphae into the surrounding liquid. Many of these hyphae later serve as evacuation tubes in the discharge of extensively ramified sporangial units. Often the evacuation hyphae become modified for dehiscence not only at the axial tip (PI. XVI, A, a; B, a) but also at the tips of one or more short branches or spurs (PL XVI, A, b, c; B, b) borne laterally in distal positions, though not all of the available tips need actually operate in discharging the zoospores. In instances where the axial tip and a single lateral spur would seem to offer equally favorable passageways the axial tip (PL XVI, C, a) sometimes remains closed, with the result that all the zoospores escape through the open spur (PL XVI, C, b), whereas at other times the lateral spur remains closed (PL XVI, D, b) and all the zoospores are released from the open axial tip (PL XVI, D, a). Evacuation tubes with five (PL XVI, E, a—e) or six (PL XVI, F, a—f) available tips may liberate zoospores from several openings (PLXVI, E, a, b, c, e; P, a, b, d, e) even should one (PLXVI, E, d) or more (PL XVI, P, c, f) of the tips remain closed.
XVI, J, a—d) or of more than ordinary (PL XVI, J, e, f) size, often become rather strongly vacuolated and noticeably larger. Zoospores germinating soon after their encystment, and in water almost devoid of nutrients, commonly extend a germ hypha only 2 to 2.5 ^ wide (PL XVI, K—0). Though yielding encysted zoospores in enormous numbers all the Aphanomyces cultures I have isolated from pansies have been reluctant to produce motile zoospores. No motile zoospores whatever came under observation in the irrigated material of the Bethesda isolation used in making the drawings of asexual reproductive apparatus shown in PL XVI. Pronounced reluctance of A. cladogamus to form motile zoospores under conditions highly suitable for development of the motile stage in congeneric root-rotting parasites was noted in my description of the species (D r e c h s 1 e r, 1929), and more recently was observed likewise by M c K e e n (1952) in isolations he obtained from pepper (Capsicum frutescens L.) seedlings that had damped-off under glass.
The pansy Aphanomyces readily infects seedlings of the three vegetable crop plants — tomato, pepper, and spinach — on which A. cladogamus has been found to occur spontaneously. Seed of the three vegetables was sown in sterilized dishes filled with sterilized sand through which had been distributed small pieces of maize-meal agar well permeated with vigorous mycelium of the Bethesda isolation, each 100 square centimeters having received an admixture of about 1 gram of permeated agar. Distilled water free of alien oomycetes was used to keep the sand continuously moist. Approximately one-third of the tomato seedlings that emerged succumbed to 00* damping-off. Fully three-fourths of all pepper seedlings damped-off either before or after emergence. Less than one-tenth of all spinach seedlings escaped destruction. When infected seedlings were placed on maize-meal agar plates they always yielded Aphanomyces mycelium very promptly. In the tests spinach showed generally such pronounced liability to attack by the pansy Aphanomyces as to suggest that the fungus may at times cause serious damage to young stands of this vegetable in garden or field. Eggplant (Solanum melongena L.) seedlings were included in the inoculation trials, and were found to succumb to invasion by the pansy Aphanomyces in about the same measure as pepper seedlings.
In view of very close agreement with respect to biological relationships as well as with respect to morphology of both sexual and asexual reproductive apparatus the Aphanomyces found associated with severe root rot of pansies in and near the District of Columbia is held to be identical with A. cladogamus.
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Explanation of plates XI—XVI Plate XI. Pansy plants (among other ornamentals) photographed on May 19, 1933, in a garden in Mount Rainier, Maryland, from which had been obtained 20 days earlier many pansy specimens with extensive red stele infection of their root systems. A, Badly wilted pansy (near center) between apparently healthy specimens on left and right. B, Severely withered pansy (left) near an apparently healthy one (right).
Plate XII. Pansy plants (among other ornamentals) photographed on May 19, 1933, in a garden in Mount Rainier, Maryland, from which had been obtained 20 days earlier many pansy specimens with extensive red stele infection of their root systems. A, Severely withered pansies (near center) between pansy specimens (lower left and upper right) showing only incipient wilting. B, Dead remains of a pansy (lower right) that succumbed to severe root rot at a distance only about 50 cm. from a plant (left) apparently still healthy.
Plate XIII. Sexual reproductive apparatus of an isolation of Aphanomyces cladogamus obtained in May, 1939, from a decaying root of a diseased pansy taken from a garden in Arlington, Virginia; all reproductive units produced on the under side of maize-meal agar plate cultures, and drawn to a uniform magnification with the aid of a camera lucida; X 1000 throughout. A—C, Diclinous reproductive units with one mycelial hypha (a) giving off the oogonial stalk, and another hypha (b) supplying the branches bearing the plural antheridia; oospore immature in A, mature in B and C. D—H, Mature monoclinous reproductive units, showing close mycelial connection between the hyphal branch (a) supplying the oogonium and the hyphal branch (b) supplying the male complement consisting of one (G) or more (D, E, F, H) antheridia.
Plate XIV. Sexual reproductive apparatus of an isolation of Aphanomyces cladogamus obtained in May, 1939, from a decaying root of a diseased pansy taken from a garden in Arlington, Virginia; all reproductive units formed on the under side of maizemeal agar plate cultures, and drawn to a uniform magnification with the aid of a camera lucida; X1000 throughout. A—D, Diclinous reproductive units, with one mycelial hypha (a) supplying the oogonium, and another hypha (b) supplying the male complement consisting of one (C, D) or more (A, B) antheridia. E, F, Monoclinous reproductive units showing close mycelial connection between the hyphal branch (a) supplying the oogonium and the hyphal branch (bi supplying the male complement consisting of one (F) or more (E) antheridia. G, H, Monoclinous reproductive units in each of which the hyphal branch (a) supplying the oogonium is given off from the same mycelial filament from which arise also the 2 branches, b—c, that together supply the attendant antheridia. Oospore slightly immature in E and H;
fully mature in A, B, C, D, F.
Plate XV. Sexual reproductive apparatus of an isolation of Aphanomyces cladogamus obtained from decaying stem tissue of a collapsing pansy taken from a garden near Bethesda, Maryland, on May 23, 1953; all reproductive units formed on the under side of maize-meal agar plate cultures, and drawn to a uniform magnification with the aid of a camera lucida; X1000 throughout. A, Diclinous reproductive unit, with one mycelial hypha (a) supplying the oogonium, and another hypha (b) supplying the two attendant antheridia. B—G, Monoclinous reproductive units in each of which the same mycelial filament gives off both the hyphal branch (a) supplying the oogonium and the hyphal branch (b;
supplying the one (C, D, E) or two (B, F, G) attendant antheridia. Oospore slightly immature in B and C, but in fully mature resting condition in A, D, E, F, G.
Plate XVI. Asexual reproductive apparatus of the same isolation of Aphanomyces cladogamus as is shown in Plate XV; all parts formed in irrigated Lima-bean agar preparations and drawn with the aid of a camera lucida. A, Terminal portion of evacuation tube about 15 minutes before sporangia! discharge, showing three tips (a—c) available for dehiscence;
X 500. B, Terminal portion of evacuation tube about 2 minutes before sporangial discharge, showing two tips (a, b) available for dehiscence;
X 500. C, Same after discharge of sporangium, showing axial tip (a) closed and lateral tip (b) open; X500. D, Distal portion of another evacuation tube after discharge of sporangium, showing axial tip (a) open and lateral tip (b) closed; X500. E, Distal portion of evacuation tube in late stage of sporangial discharge, showing four open (a—c, e) and one closed (d) tip; X500. F, Distal portion of evacuation tube in late stage of sporangial discharge, snowing four open (a, b, d, e) and two closed (c, f) tips; X500.
G, Newly encysted zoospores (a—z) of usual size; X500. H, Newly encysted oversized zoospores (a, b) resulting from imperfect cleavage in sporangium; X500. I, Newly encysted zoospores (a—p) of usual size; X1000.
J, Zoospores of usual (a—d) and of larger than usual (e, f) sizes, showing pronounced vacuolization and enlargement two days after their encystment;
X1000. K—0, Zoospores germinating in water; X500. (Owing to lack of space E and F are shown in parts whose proper connection is indicated by broken lines.) Plate XI.
Sydowia. — Annal. Mycol. Ser. II. Vol. VIII. 105 Xyrluwiu. — Annul. Myuol. Ser. II. \ 7 ol. VIII. 1954. 1'late XII.
Syduwiu. — Aunal. Mycol. Sef. II. Vol. VIII. 11)54. Plate XIIL Sydowiu. — Annal. Mycol. Ser. II. Vol. VIII. 1954. Plate XIV.
Sydowia. — Anna!. Mycol. Ser. II. Vol. VIII. 195i. Plate XV.
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