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Rust pustules support invasion of ramsons by Botrytis cinerea
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RUST PUSTULES SUPPORT INVASION OF RAMSONS BY BOTRYTIS CINEREA S G HALLETT AND N D PAUL Division of Biological Sciences, Institute of Environmental &' Biological Sciences, University of Lancaster, Bailrigg, Lancaster LAl 4YQ Few plant pathogens are more conspicuous than the rust fungi and one of the more obvious in the UK is the aecial stage of Puccini a sessilis Schroeter, which may be found each spring on the dark green leaves of ramsons (Allium ursinum). This rust has been observed for several years on isolated ramson populations, growing in woodland alongside the Lancaster canal where the secondary host, Phalaris arundinacea, grows in abundance. In most years, the rust occurs only as scattered pustules but in the spring of 1988 a more widespread infection occurred in several ramson populations and, in April, a sudden and severe die-back of rust-infected plants occurred. Figure A shows a rust population of ransoms prior to this die-back, and Figure B shows a later stage in which the grey-brown necrosis originating from rust pustules has spread over much of the leaf. Eventually, within approximately four weeks, a high percentage ofthe rusted shoots in the population were killed. The symptoms were associated exclusively with rust pustules, rust-free leaves remaining unaffected. Figure C shows the under-side of a ramson leaf bearing three rust pustules. One pustule lies at the centre of a spreading necrotic region showing concentric light and dark bands of which the former appeared to correspond with regions of profuse sporulation of a secondary pathogen. Figure D shows a similar effect with the leaf on the left-hand side showing a necrotic region spreading up the midrib from its origin at the rust
pustule. These symptoms suggested that rust pustules have acted as a point of infection for a secondary pathogen, and when lesions were cultured on malt agar, a profuse mycelium developed, which was identified at CMI as Botrytis cinerea Pers. When conidia from these cultures were applied back to rusted ramsons in the field, symptoms identical to natural infections resulted. By contrast, inoculation of conidia onto non-rusted leaves caused no visible symptoms. Thus P. sessilis and B. cinerea in combination resulted in devastating injury although neither caused severe damage alone. The possible mechanisms of this synergy are very interesting. The rust, by breaching the epidermis, will provide a wound through which opportunistic pathogens, such as B. cinerea, can enter and infect the host, further growth being favoured by solutes accumulated in and around rust pustules. The possibility exists that non-specific host resistance by rust may also be important in allowing unrestricted growth of the necrotroph. A number of reports of severely damaging secondary infections by necrotrophs spreading from sites of rust (Walker, 1921; Dimock & Baker, 1951; Yarwood, 1976; Paul & Ayres, 1986) or mildew infection (Brokenshire, 1974; Forrer, Rijsdijk & Zadoks, 1982) . demonstrate that the phenomenon is not confined to this particular example involving ramsons. Many more similar phenomena on crop and wild plants may have been overlooked. Work at Lancaster
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on Senecio spp is seeking to exploit such interactions in the biological control of weeds. This potentially useful interaction emphasises the benefits which may result from a better understanding of disease in natural vegetation. REFERENCES
BROKENSHIRE, T (1974). Predisposition to Septoria infection following attack by Erysiphe. Transactions of the British Mycological Society 63, 393-397 . DIMOCK, A W & BAKER, K F (1951). Effect of climate on disease development, injuriousness and fungicidal control as ex-
emplified by snapdragon rust. Phytopathology 41 , 536-552 . FORRER, H R, RIISDlIK , F H & ZADOKS, J C (1982) . Can mildew assist in the entry of Fusarium fungi into wheat leaves ? Netherlands Journal of Plant Pathology 88, 123-125. PAUL, N D & AYRES, P G (1986) . Seasonal effects on rust disease (Puccinia lagenophorae) of Senecio vulgaris . Symbiosis 2 , 165-173. WALKER, J C (1921). Rust of onion followed by a secondary parasite Phytopathology 11, 87-90. YARWOOD, C E (1976). Pseudoperonospora cubensis in rust-infected bean . Phytopathology 67 , 1021·1022.
Fig . (A) Rust-infected stand of ransoms; (B) die-back of ransoms due to secondary infection of B. cinerea; (C) under-side of ransom leaf showing three rust pustules, with a spreading lesion caused by B. cinerea shown on left; (D) under-side of ransom leaves with rust pustules (right) and a lesion of B. cinerea spreading up midrib from its origin at a rust pustule (left) .
RUST PUSTULES SUPPORT INVASION OF RAMSONS BY BOTRYTIS CINEREA S G HALLETT AND N D PAUL Division of Biological Sciences, Institute of Environmental &' Biological Sciences, University of Lancaster, Bailrigg, Lancaster LAl 4YQ Few plant pathogens are more conspicuous than the rust fungi and one of the more obvious in the UK is the aecial stage of Puccini a sessilis Schroeter, which may be found each spring on the dark green leaves of ramsons (Allium ursinum). This rust has been observed for several years on isolated ramson populations, growing in woodland alongside the Lancaster canal where the secondary host, Phalaris arundinacea, grows in abundance. In most years, the rust occurs only as scattered pustules but in the spring of 1988 a more widespread infection occurred in several ramson populations and, in April, a sudden and severe die-back of rust-infected plants occurred. Figure A shows a rust population of ransoms prior to this die-back, and Figure B shows a later stage in which the grey-brown necrosis originating from rust pustules has spread over much of the leaf. Eventually, within approximately four weeks, a high percentage ofthe rusted shoots in the population were killed. The symptoms were associated exclusively with rust pustules, rust-free leaves remaining unaffected. Figure C shows the under-side of a ramson leaf bearing three rust pustules. One pustule lies at the centre of a spreading necrotic region showing concentric light and dark bands of which the former appeared to correspond with regions of profuse sporulation of a secondary pathogen. Figure D shows a similar effect with the leaf on the left-hand side showing a necrotic region spreading up the midrib from its origin at the rust
pustule. These symptoms suggested that rust pustules have acted as a point of infection for a secondary pathogen, and when lesions were cultured on malt agar, a profuse mycelium developed, which was identified at CMI as Botrytis cinerea Pers. When conidia from these cultures were applied back to rusted ramsons in the field, symptoms identical to natural infections resulted. By contrast, inoculation of conidia onto non-rusted leaves caused no visible symptoms. Thus P. sessilis and B. cinerea in combination resulted in devastating injury although neither caused severe damage alone. The possible mechanisms of this synergy are very interesting. The rust, by breaching the epidermis, will provide a wound through which opportunistic pathogens, such as B. cinerea, can enter and infect the host, further growth being favoured by solutes accumulated in and around rust pustules. The possibility exists that non-specific host resistance by rust may also be important in allowing unrestricted growth of the necrotroph. A number of reports of severely damaging secondary infections by necrotrophs spreading from sites of rust (Walker, 1921; Dimock & Baker, 1951; Yarwood, 1976; Paul & Ayres, 1986) or mildew infection (Brokenshire, 1974; Forrer, Rijsdijk & Zadoks, 1982) . demonstrate that the phenomenon is not confined to this particular example involving ramsons. Many more similar phenomena on crop and wild plants may have been overlooked. Work at Lancaster
175
on Senecio spp is seeking to exploit such interactions in the biological control of weeds. This potentially useful interaction emphasises the benefits which may result from a better understanding of disease in natural vegetation. REFERENCES
BROKENSHIRE, T (1974). Predisposition to Septoria infection following attack by Erysiphe. Transactions of the British Mycological Society 63, 393-397 . DIMOCK, A W & BAKER, K F (1951). Effect of climate on disease development, injuriousness and fungicidal control as ex-
emplified by snapdragon rust. Phytopathology 41 , 536-552 . FORRER, H R, RIISDlIK , F H & ZADOKS, J C (1982) . Can mildew assist in the entry of Fusarium fungi into wheat leaves ? Netherlands Journal of Plant Pathology 88, 123-125. PAUL, N D & AYRES, P G (1986) . Seasonal effects on rust disease (Puccinia lagenophorae) of Senecio vulgaris . Symbiosis 2 , 165-173. WALKER, J C (1921). Rust of onion followed by a secondary parasite Phytopathology 11, 87-90. YARWOOD, C E (1976). Pseudoperonospora cubensis in rust-infected bean . Phytopathology 67 , 1021·1022.
Fig . (A) Rust-infected stand of ransoms; (B) die-back of ransoms due to secondary infection of B. cinerea; (C) under-side of ransom leaf showing three rust pustules, with a spreading lesion caused by B. cinerea shown on left; (D) under-side of ransom leaves with rust pustules (right) and a lesion of B. cinerea spreading up midrib from its origin at a rust pustule (left) .