Agriculture Reference
In-Depth Information
break down the prunasin in the region of the graft union to give hydrocyanic
acid as one of the decomposition products. Different pear cultivars vary in
their content of a water-soluble inhibitor of the action of the enzyme which
catalyses the breakdown of prunasin. The presence of hydrocyanic acid at the
graft union checks cambial activity there and also destroys phloem tissues at
and above the union. Conduction of water and materials through the union is
reduced. The reduction in the amount of sugars reaching the quince roots
leads to further decomposition of prunasin, liberating hydrocyanic acid and
killing quince phloem (Gur,
).
Further evidence that graft incompatibility between pear and quince involves
the influence of cyanogenic glycosides produced by the quince has come from
suspension culture studies. These showed that pear callus growth was inhib-
ited in media in which quince cultures had been grown, and addition of a
cyanogenic glycoside killed pear cultures (Moore,
; Gur and Samish,
; Gur
et al.
,
). Moore concluded
that, given this mechanism, incompatibility between pear and quince need
not be associated with any particular stage of graft development. Other stud-
ies (Buchloh,
) showed failure of lignification in incompatible pear-quince
unions.
Garner (
cultivars of pear which show some degree of incom-
patibility with quince but will form more reliable unions if they are worked
on an intermediate which is worked on the quince. The most widely used
intermediates are 'Beurre Hardy' and 'Old Home'. The most important cul-
tivars that are not directly compatible with quince are 'Williams' ' ('Bartlett'),
most Asian pears, 'Forelle', 'Bosc' and 'Jules Guyot'. Important compati-
ble cultivars include 'Comice', 'Passe Crassane', 'Beurre Hardy', 'Clapps
Favourite' and 'Conference'. Some selections of 'Williams' ' are compatible
with quince.
'M.
) lists
' is the only important apple rootstock to have shown incompatibility
with a number of scions.
. So-called virus-induced incompatibility affects both apples and pears.
Union necrosis and decline of apple is caused by tomato ringspot virus which
is transmitted by the soil-borne nematode
Xiphinema americanum
. Trees infected
with this break at the union, with a clean break at least on one side. The
rootstocks 'M.
' and 'MM.
' and the scion cv. 'Delicious' seem susceptible,
while 'M.
', 'Empire' and 'Golden Delicious'
appear to be tolerant (Ferree and Carlson,
', 'M.
', 'Ottawa
', 'Robusta
). Pear decline is caused by
a phytoplasma (mycoplasma-like organism, MLO; see Chapter
)whichis
spread by the insect
Cacopsylla pyricola
(Hibino and Schneider,
; Hibino
et al.
,
). This may cause little direct damage to a scion cultivar but as
the phytoplasma moves across the graft union into the tissues of a susceptible
rootstock (especially
P. ussuriensis
or
P. pyrifolia
) the rootstock phloem is killed, the
