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are largely resistant to V. album, and that American oaks also display a restricted receptivity.

Apart from being relatively rare, mistletoe-bearing oaks also tend to bear only a small number of mistletoe bushes. It is interesting to compare the number of mistletoe plants found on the following 4 groups of rare host trees (Figure 1):

- indigenous oaks (Q. robur and Q. petraea)

- Q. palustris/coccinea

Figure 1 Incidence of different Viscum album colonization densities on different host trees (Q. robur/petraea, Q. rubra, Q. palustrislcoccinea and Ulmus sp.) in France plotted against percentage of host trees (% of total number investigated), n=no. of trees investigated.

Three quarters (74.0%) of all mistletoe-bearing indigenous oaks (Q. robur and Q. petraea) bear only 1-5 mistletoe bushes. The corresponding percentages are also high for Q. rubra, Q. palustris/coccinea and Ulmus sp. at about 50%. The histograms in Figure 1 show that the trees become all the rarer the higher the number of mistletoe plants they bear. Q. robur/petraea and Q. rubra differ clearly from Q. palustris/coccinea and Ulmus sp. in the frequency of individuals carrying large numbers of mistletoe plants. Mistletoe numbers of 21 to 100 bushes are four times less common with the former (5.8%) than the latter (22.1%).

Experimental sowing of mistletoe seed on wild-growing indigenous mistletoe-bearing oaks showed that about 40% of the trees would not accept new mistletoe plants, even if repeated sowings were made (Table 1). The mistletoe seed used was investigated and did not show reduced vitality. It is probable, therefore, that these mistletoe-bearing oaks had an inherent resistance. This indicates that receptivity might be expressed only during a limited phase of the trees' life.

Mistletoe oaks on which it proved possible to establish new mistletoe plants showed a correlation between receptivities and the number of preestablished native mistletoe bushes (Table 1). Oaks bearing only few native bushes would as a rule only accept a few additional mistletoe bushes, whereas the proportion of trees accepting many (>10) new mistletoe plants was high within the group of oaks with many native bushes (Table 1).

Differences in mistletoe frequencies on oaks, therefore, are unlikely to be due only to differences in colonisation pressure from mistletoe. Differential expression of receptivity might be the major cause for the observed differences. A number of structural elements in oak and poplar bark have been identified as definite resistance parameters. A correlation was in fact established between a resistance coefficient taking account

Table 1 Success rate of artificial V. album sowings on native oaks (Q. robur and Q. petraea).

No. of mistletoe

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