Articles
EVALUATION OF PYRUS GERMPLASM FOR RESISTANCE TO THE PEAR PSYLLA
Do you know something about this resistance ?
R.L. BELL – We have not evaluated "SIRRINE" for resistance to pear psylla. "MOONGLOW" was, however, found to be only moderately susceptible.
It is not as resistant as the germplasm of Pyrus ussuriensis parentage.
B. THIBAULT – What is the parentage of :
| NY | 10358 |
| 10351 | |
| 10352 |
R.L. BELL – The selections, NY 10351 and NY 10352, are first generation backcross (BC1)hybrids of Pyrus ussuriensis and Pyrus communis, and are, therefore, 75% P. communis. The original parents were high quality European cultivars and the selection Illinois # 65.
The selection NY 10358 is of Pyrus communis parentage.
S.J. WERTHEIM (Netherlands) – Why do psylla nymphae not survive on resistent pear cultivars, especially after their third stage ?
R.L. BELL – There is no proven answer to this question at the present time.
We may speculate however on the basis of some related research.
Fiori and Lamb found a positive correlation between psylla resistance, based upon counts of nymphs on field grown trees, and the percentage of the phloem in leaf mid-rib veins occupied by tannin-containing cells.
This correlation was found early in summer, due to a decrease in the number or area of the tannin cells.
It has been shown by work in McLean’s laboratory at the university of California at Davis that the psylla nymphs are phloem feeders.
Because the mortality of the nymphs is cumulative, with few surviving to the fourth and fifth instar, one may speculate that the mechanism of antibiosis involves something ingested by the psylla, perhaps the tannins or some other substance in the phloem sap, or the lack of some substance necessary for nutrition, or a reduced level or lack of a chemical "signal" needed to initiate feeding by the nymphs.