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Fusarium wilt-resistant ‘Novada’ and wilt-susceptible ‘Early Sam’ carnations were planted in soil infested withFusarium oxysporum f.sp.dianthi, and their roots studied after five and ten weeks. Both in ‘Novada’ and ‘Early Sam’, the extravascular tissue of undamaged young root parts were scarsely colonized. In roots of ‘Novada’, infected xylem vessels were usually occluded with gums and surrounded by phellem tissue. In mature parts of roots, the phellem surrounding occluded vessels often merged with the external phellem surrounding the vascular cylinder, after which the occluded vessels were shed from the roots. The phellem at the root surface appeared to be a strong barrier to fungal invasion. In roots of ‘Early Sam’ carnations, as well as in a few roots of ‘Novada’ carnations, the defence responses did not result in compartmentation of the fungus, and colonization and degradation of the vascular tissues followed. Diseased roots finally healthy. Shoots of ‘Early Sam’ carnations, and eventually a few shoots of ‘Novada’ carnations, were colonized and developed wilt symptoms.

Sterol biosynthesis inhibitors that inhibit the yeast-hyphae conversion inOphiostoma ulmi suppressed Dutch elm disease development in two elm clones. After curative treatment with fenpropimorph-sulphate of 27 ‘Vegeta’ elms which had previously been inoculated withO. ulmi, 25 trees did not show disease symptoms by the end of the second season. All 41 control trees, inoculated withO. ulmi only, were clearly diseased. In an experiment with ‘Commelin’ elms three fenpropimorph salts and thiabendazole were compared. Injection of the trees three weeks after inoculation withO. ulmi gave by the end of the second season no symptoms of Dutch elm disease in any of the trees injected with fenpropimorph-phosphate or thiabendazole, and in most trees injected with fenpropimorph-acetate or-sulphate. Similar treatments with the free base of fenpropimorph and fenpropidin-sulphate were less effective due to insufficient uptake of the fenpropimorph emulsion and phytotoxicity of fenpropidin-sulphate, respectively. Injection of fenpropimorph-sulphate or thiabendazole six weeks after inoculation withO. ulmi did not result in significant differences from the control group inoculated withO. ulmi only.

Susceptible interactions of ‘Early Sam’ carnations with races 1,2,4, and 8 ofFusarium oxysporum f. sp.dianthi differed in pathogenesis, both after stem and after root inoculation. Race 1 induced pallescence and withering of leaves. Affected vascular tissue had a uniform pallid to pale brown colour; though heavily colonized, it was not or virtually not degraded. Defence reactions developed only slowly. Race 2 induced yellowing, of the midribs in particular, and withering of leaves. Affected vascular tissue was white with dark brown margins. Colonized tissue was degraded to leave vascular cavities. At lower heights of colonization, many defence reactions developed, which sometimes resulted in localization of the pathogen. Race 4 induced a similar pathogenesis as race 2, except for less intensive defence reactions. Race 8 induced midrib lesions on, and pallescence, withering and necrosis of leaves. Affected vascular tissue had a uniform light brown colour. Degradation of colonized vascular tissues was rare; instead, many defence reactions were observed, even at high heights in the plants.

Stems of the susceptible ‘Early Sam’ and resistant ‘Novada’ carnations were inoculated with a conidial suspension ofFusarium oxysporum f. sp.dianthi. Stem segments of either cultivar were sampled regularly and used for determination of fungal growth and for microscopical investigation.

The c10 phenolic acids, 2,4-dihydroxy-6-acetonylbenzoic acid (1) and its 6-(1-hydroxyacetonyl) (2) and 6-pyruvyl (3) analogues, metabolic products ofO. ulmi on a chemically defined medium are also produced on elm tissue medium but are rapidly metabolised once the carbon source is exhausted. The phytotoxicity to cut shoots ofU. glabra andU. hollandica of these acids and their 4-0-methyl ethers (4–6 respectively) and of 4-hydroxyphenylacetic acid (7) has been investigated. None of the C10 acids consistently caused wilting or leaf necrosis inU. glabra but all three were toxic toU. hollandica. Diaporthic acid (4) caused severe leaf necrosis in bothUlmus sp. and the phenolic acid (7) caused wilting and necrosis ofU. glabra shoots. Attempts to reproduce these effects in rootedU. glabra saplings were unsuccessful. Autoradiography ofU. glabra shoots treated with [14C]-labelled acids (2) and (4) showed a uniform distribution of radio-activity in the tissues, but the labelled acids could not be recovered. There is no evidence that the low molecular weight metabolic products ofO. ulmi are toxic to rooted saplings or proof that they are formed in vivo.

Spread of an aggressive and a non-aggressive strain ofOphiostoma ulmi (Buisman) Nannf. in the susceptible elmUlmus hollandica cl. Belgica was found to be similar after inoculation with a mixture of conidia of both strains. Survival of the aggressive strain in this clone was better than that of the non-aggressive strain. InU. hollandica cl. 390 and the ‘Christine Buisman’ elm, both resistant to the non-aggressive strain, spread of the non-aggressive strain was limited after inoculation of a mixture of conidia of both strains and the survival of the aggressive strain was also favoured over that of the non-aggressive one.

The indirect immunofluorescence technique (IIF) and the indirect immunoperoxidase technique (IIP) were used to detect specificallyOphiostoma ulmi in inoculated elm branches. A strong cross-reaction was observed between the hyperimmune serum to the non-aggressive strain E2 ofO. ulmi and the aggressive strain H6. No reactions were found withVerticillium albo-atrum orFusarium oxysporum.

Different wood preparation techniques for a SEM study of the growth ofOphiostoma ulmi (Buisman) Nannf. in elm tissue were compared. Critical point drying appeared to be the most suitable method. All observations were made 10–14 cm above the site of inoculation. In the resistant elm,Ulmus hollandica cl. 390, inoculated with a non-aggressive strain ofO. ulmi, no hyphae could be found until 5 days after inoculation. However, whenU. hollandica cl. 390 was inoculated with the aggressive strain, the fungus could already be observed after 3 days. In the susceptible elm,U. hollandica cl. Belgica, whether inoculated with an aggressive or non-aggressive strain, hyphae were found within 2 days after inoculation. The fungus appears only to penetrate the vessel pits and has at no time been seen to penetrate the cell wall directly.

For biological control of Dutch elm disease byPseudomonas spp. a population of these bacteria has to be present within the tree. Gaining insight in the distribution of such bacteria, both spatially within the tree and over time, was one goal of this study, determining limiting factors of the habitat and selection bacteria better adapted to it, the other.

No evidence was found that mansonones E and F, which accumulated in elms after infection withCeratocystis ulmi, were responsible for resistance against Dutch elm disease inUlmus hollandica cl. 390. Furthermore, production of mansonones inU. americana was only about one-fifteenth of that inU. hollandica cl. ‘Belgica’. Yet both are susceptible to Dutch elm disease.

Conductivity of the vascular system of 2-year-old shoots as regards air and water was higher in susceptible than in resistant elms. The xylem vessels of resistant elms were relatively shorter and the percentage of short vessels was greater than in susceptible elms. The percentage of vessels with a large diameter was smaller in resistant elms than in susceptible ones. These results might explain the limited spreading of the spores in the new annual ring of resistant elms.

Samenvatting De vatbare iepUlmus hollandica kl. Belgica en de voor de niet-agressieve stam resistante iepU. hollandica kl. 390 werden geïnoculeerd met mengsels van condia van een agressieve en een niet-agressieve stam vanOphiostoma ulmi. Kloon Belgica vertoonde in alle gevallen ernstige symptomen, maar bij kloon 390 werd een geringere symptoomontwikkeling waargenomen naarmate de hoeveelheid conidia van de niet-agressieve stam in het mengsel toenam. Kennelijk induceert de niet-agressieve stam een mechanisme in deze plant, dat althans gedeltelijk de inductie of expressie van ziektesymptomen veroorzaakt door de agressieve stam, tegengaat.

Samenvatting De hypothese dat de thyllenvorming in de houtvaten van de iep als reactie op een infectie metOphiostoma ulmi alleen in de compatibele waardplant-pathogeen combinatie vertraagd zou zijn, werd getoetst. Er bleek geen significant verschil in snelheid van thyllenvorming op te treden als de vatbare kloon Belgica geïnoculeerd werd met een agressieve en een niet-agressieve stam vanO. ulmi of metFusarium oxysporum f. sp.lycopersici, een niet-pathogeen voor iepen. Overeenkomstige resultaten werden verkregen wanneer kloon 390, die resistent is tegen de niet-agressieve stam vanO. ulmi, geïnoculeerd werd met deze drie schimmels. In deze gevallen wordt de snelheid van thyllenvorming alleen bepaald door de aard van de betreffende iepekloon.

After inoculation of elms withCeratocystis ulmi tylose formation in vessels of the susceptible clone ‘Belgica’ appeared to be delayed in comparison with tylose formation in the resistant clone ‘390’. It is suggested that tylose formation may be a resistance mechanism in elms to Dutch elm disease.

Samenvatting Een week na inoculatie van vatbare tomateplanten metVerticillium albo-atrum werd uit het vaatweefsel een fytoalexine geïsoleerd, dat idententiek bleek te zijn aan het polyacetyleen cis-tetradeca-6-een-1,3-diyn-5,8-diol.