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.Data of resistance mechanisms of elms to Ceratoeystis ulmi (Buisman) C. Moreau are scarce. Pope (1943) found some anatomical differences between the susceptible Ulmus americana L. and the resistant U. pumila. In the latter the vessels of the springwood were smaller than in the former. Frequently in U. pumila only one row of these vessels was found. In this elm species formation of summerwood took place earlier in the growing season than in U. americana. In the latter the amount of branching between different vessel bundles, both radially and tangentially appeared to be more frequent than in U_ pumila, which might result in a more extensive spreading of the spores of C. ulmi after infection in U. americana than in U. pumila. In both species, however, the summerwood was characterized by a lack of interconnections between vessels, limiting the distribution of the spores.More recently Elgersma (1967) reported that the resistance in U. hollandica cl. 390 was due to a limited spreading of the fungus in the vascular system. As rate of multiplication of spores in the vessels of susceptible and resistant elms appeared to be similar during the first 3 days after inoculation, anti-fungal substances did not seem to be a factor in resistance. It was suggested that gum and tylose formation might occlude the infected vessels faster in resistant elms than in susceptible ones. By this occlusion transport of spores is hampered. No difference in rate of blockage of vessels, however, could be traced between susceptible and resistant elms during the first 3 days after inoculation examining fixed plant material (Elgersma, 1969). So a faster blockade of vessels as resistance mechanism appears not to be evident.Another possibility may be, as was supposed at the First International Symposium on Dutch elm disease in 1968 (Elgersma, in press), that differences in anatomical structure such as vessel length and diameter might be important in limiting the transport of the spores in resistant elms. In clones with relatively short vessels the fungus has to penetrate more mechanical barriers than in clones with relatively long vessels, thus giving the plants the opportunity to produce an effective barrier of gums and 179

Stems of the susceptible 'Early Sam' and resistant 'Novada' carnations were inoculated with a conidial suspension of Fusarium oxysporum f. sp. dianthi. Stem segments of either cultivar were sampled regularly and used for determination of fungal growth and for microscopical investigation.'Early Sam' showed typical Fusarium wilt symptoms and its stems were colonized intensively.The observed vascular browning appeared to be caused by discolouration of primary walls of infected vessels and surrounding cells. Vessels were rarely occluded with gel. Cell wall degradation led to the formation of stem cavities. Hyperplasia of xylem parenchyma was not seen.In 'Novada', fungal colonization remained low throughout the experiment. Macroscopic symptoms were absent except for longitudinal bursts in the stem, which appeared to be caused by hyperplasia of xylem parenchyma bordering infection. Vascular gelation occurred in the infected tissues, causing some vascular browning also. Xylem vessel regeneration was observed in the hyperplastic layer. Cavities were not formed, and wall discolouration was rare. Vascular gelation is considered part of the Fusarium wilt resistance mechanism. It is followed by xylem vessel regeneration, which expresses a general plant response to vascular dysfunction rather than being part of the resistance mechanism.Although of different origin, vascular browning as such occurs in both susceptible and resistant inr.eractions. In breeding for resistance, care should hence be taken with the current use of browning as an indication of disease.

Host responses of elms susceptible and resistant to Dutch elm disease were histologically examined. In a time course study the susceptible elm clone Ulmus × hollandica 'Belgica' and U. × hollandica '390', a clone which shows a high degree of resistance to non-aggressive isolates and a moderate degree of resistance to aggressive isolates of Ophiostoma ulmi, were inoculated in twig or trunk with either an aggressive or a non-aggressive isolate of O. ulmi. For purposes of comparison, the susceptible elm U. americana and the more resistant clones U. × hollandica 'Groeneveld', U. 'Lobel' and U. 'Sapporo Autumn Gold' were included. Depending on clone-isolate compatibility, infected twigs reacted by a walling off process, by barrier zone formation, or failed to resist the infection and died. Trees inoculated into the trunk reacted comparably but in the case of a compatible combination they always formed a barrier zone and the cambium never died in the year of inoculation.

Tomato plants, susceptible toFusarium oxysporum f. sp. lycopersici, were inoculated by immersing the roots in a conidial suspension ofF.

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