Cucurbit Genetics Cooperative Report 7:81-83 (article
Further Notes on the Silvery-Leaf Trait in Cucurbita
Department of Horticulture and Forestry, Cook College, Rutgers
University, New Brunswick, NJ 09803
The silvery-leaf trait (6) raises two issues: the biological
control of its complex expression and its adaptive value. The
biological control is poorly understood. But there is little
doubt that, under field conditions in New Brunswick, inbreds of C. pepo L. differ greatly in (i) time during plant
development at which trait is first manifested, (ii) intensity of its expression, and (iii) extent of
its distribution over the upper leaf surface (5). The silvery
expression can be confined to veins and/or vein axils (mottled-
leaves) or can be extended over the entire leaf surface.
Five different forms, SL-1 to SL-5, have been described (6).
Representatives of SL-1 are 'Early Prolific Straightneck' (EPS)
and 'Jersey Golden Acorn'. These cultivars are assumed to carry m/m (m for non-mottled or non-silvery leaves) as
well as some modifiers of M (M for mottled or
silvery leaves) which have small silvery effects of their own.
Representatives of SL-2 to SL-5 are assumed to be M/M lines which differ from one another in their modifiers or
controllers. The most extreme form, SL-5, is represented by
NJ260. This inbred can produce uniformly silvery leaves of
intense expression throughout plant life.
Several new intermediate (between SL-2 and SL-5) inbreds were
developed from crosses involving NJ260. In addition, two
cultivars were found to differ from previously described forms:
'Bicolor Spoon' and 'Delicata'. Our inbred of 'Bicolor Spoon' is
truly green or non-silvery. Hypothetically, it carries M/m but lacks the modifiers which have small silvery
effects. As to 'Delicata', Tapley et al. (8) described its
leaves as "silvery green". Under our field conditions,
'Delicata' produces green leaves during spring and midsummer, and
uniformly silvery leaves in late summer and fall. The silvery
expression in 'Delicata' often spreads from the tips to the
entire surface of the leaves rather than from the veins or vein
axils. If the present hypothesis is correct, 'Delicata' carries M/M and a unique combination of controllers. But I wish I
were as confident in the hypothesis as I am in the description of
the above lines.
Non-genetic factors profoundly affect the expression of the
silvery-leaf trait. High expressivity is obtained predictably
under the following controlled conditions: 16 hr photoperiod, 11
x 103 lu/m2, 90% of which is from
fluorescent tubes and 5% from incandescent bulbs, 20% at day and
15% at night. Expressivity is very low under greenhouse (shaded)
conditions in midsummer when temperatures are often extremely
high. However, extremely high temperatures under field
conditions do not drastically reduce expressivity. These
tentative observations suggest that expressivity is modified by a
combination of non-genetic influences.
Unlike C. pepo, C. moschata (Duch.) Poir. is
largely of tropical adaptation and most, if not all, its tropical
cultivars have distinctly mottled leaves. Indeed, mottled-leaf
is one of the distinguishing features of this species in several
taxonomical treatises (e.g., in 9). An exception is a group of
Butternut cultivars (2) which probably evolved in North America.
The F2 of an interspecific cross made between C.
pepo 'Jersey Golden Acorn' (non-mottled), as seed parent, and C. moschata 'Burpee Butterbush' (non-mottled) consisted of
an appreciable number of mottled as well as uniformly silvery
segregates. These results indicate that the designation of a
single locus for control of mottled-leaf in Cucurbita (4)
is an over-simplification.
Is the silvery-leaf trait due to a neutral genotype that has been
randomly fixed in our cultivars? Alternatively, does it have a
Initial observations suggested that the silvery-leaf trait
provides an escape mechanism against aphids and aphid-
transmitted virus diseases (5). Results of a subsequent field
study supported this hypothesis, but showed that protection is
not complete (3). Circumstantially, two other facts support this
hypothesis. First, silvery leaves reflect more light than non-
silvery leaves (7), and light reflectance is generally assumed to
be involved in repelling aphids. Second, as pointed out above,
most if not all tropical cultivars of C. moschata are
mottled. Virus diseases are particularly destructive in the
tropics. Therefore, leaf mottling may have a selective advantage
if it provides some protection against aphids.
Results of a recent field experiment (1) showed that
significantly more aphids are captured on trapping stakes placed
near silvery plants than near non-silvery plants. The difference
is particularly striking in the upper trapping zone, the zone of
highest aphid concentration. The same experiment showed also
that significantly more aphids are captured on stakes placed in
bare ground than in cultivated ground occupied by plants. Since
the plants of the silvery cultivar (NJ260) used in this
experiment were smaller than those of the non-silvery cultivar
(EPS), each silvery plot consisted of larger areas of bare
ground. According to our present interpretation, this factor
together with increasing evidence for the repelling action of
silvery plants (the light effect) could have contributed to the
higher number of aphids captured near them.
It is difficult to design and interpret field experiments of this
kind. The nature of the relationship between plants and aphid
behavior is still obscure, aphid infestation is unpredictable,
and present silvery and non-silvery lines are not isogenic.
Nevertheless, the hypothesis that the silvery-leaf trait provides
and escape mechanism against aphids is potentially amenable to
- Costa, S.D. and O. Shifriss. 1982. Trapping aphids among
silvery and non-silvery lines of Cucurbita pepo L. (unpublished
- Coyne, D.P. 1970. Inheritance of mottle-leaf in Cucurbita
moschata Poir. HortScience 5:226-227.
- Davis, R.F. and O. Shifriss. 1983. Natural virus infection in
silvery and non-silvery lines of Cucurbita pepo L. Plant Disease
- Robinson, R.W., H.M. Munger, T.W. Whitaker and G.W. Bohn. 1976.
Genes of the Cucurbitaceae. HortScience 11:554-568.
- Shifriss, O. 1981. Do Cucurbita plants with silvery leaves
escape virus infection? Origin and characteristics of NJ260.
Cucurbit Genetics Coop. Rpt. 4:42-43.
- Shifriss, O. 1982. On the silvery-leaf trait in Cucurbita pepo L. Cucurbit Genetics Coop. Rpt. 5:48-50.
- Shifriss, O. 1983. Reflected light spectra from silvery and non-
silvery leaves of Cucurbita pepo. Cucurbit Genetics Coop. Rpt.
- Tapley, W.T., W.D. Enzie and G.P. Van Eseltine. 1937. The
cucurbits. Vegetables of New York, Vol. I-part IV. New York
Agricultural Experiment Station, Geneva, NY.
- Whitaker, T.W. and G.N. Davis. 1962. Cucurbits. Interscience
Publications, Inc., New York.