Seedlings of watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] are commonly affected by a partial chlorophyll deficiency that is activated by low temperatures (<20C), causing foliar symptoms and growth retardation. Cotyledons appear whitish-green, whereas the first leaves display a mosaic-like variegation consisting of scattered white flecks and patches. While this disorder is common in commercial watermelon cultivars, some land races from Zimbabwe appeared to be unaffected. From cross and backcross populations of the cold-sensitive cultivar New Hampshire Midget with the cold-resistant line PP261-1 (from PI 482261), the leaf variegation was determined to be conferred by a single recessive gene. The symbol slv (seedling leaf variegation) is assigned to this factor. The dominant allele at this same locus can be exploited for the development of new “cold-resistant” cultivars and F1 hybrids, thus providing economic gain due to earlier planting.
Passionfruit woodiness virus (PWV) can infect bean (Phaseolus vulgaris L.), causing a light and dark green foliar mosaic, veinbanding, downward curling, and plant stunting. The intensity of these symptoms can vary with the strain of the virus and cultivar, but they resemble those caused by bean common mosaic virus. In genetic populations derived from crosses and backcrosses involving cultivars that are resistant (`Black Turtle 1', `Clipper', and `RedKote') or susceptible (`Black Turtle 2', `California Light Red Kidney', and `Pioneer'), a single dominant gene conferred resistance to an Australian strain PWV-K. To this gene, the symbol Pwv (Passionfruit woodiness virus) is tentatively assigned. In plants derived from rooted cuttings of backcross populations, the same factor also conditioned resistance to three other Australian strains, PWV-Mild, PWV-51, and PWV-Tip Blight.
Bean common mosaic necrosis virus (BCMNV) includes four African strains, BCMNV-NL3, -NL-5, -NL8, and -TN1, previously considered to be members of the bean common mosaic virus (BCMV) group. Many bean cultivars resistant to BCMNV-NL8 were found to be susceptible to the other strains of the virus. `California Light Red Kidney' (CLRK) and `Carbon', resistant to BCMNV-NL8, were crossed with the susceptible cultivars Sanilac or Black Turtle 2 (BT-2). In plants of F1, F2, and reciprocal backcross populations involving CLRK × `Sanilac' or BT-2 × `Carbon', the resistance to BCMVN-NL8 was determined to be conferred by a single dominant factor. The same factor was detected in BCMNV-NL8-resistant `Great Northern 1140' and `IVT-7214, when crossed with the susceptible cultivar Stringless Refugee or BT-2.
A high level of resistance to zucchini yellow mosaic virus (ZYMV) was found in four landraces of Citrullus lanatus (Thunb.) MatSum. & Nakai (PI 482322, PI 482299, PI 482261, and PI 482308) originally collected in Zimbabwe. This resistance is specific to the Florida strain of the virus (ZYMV-FL), which prevails in the United States. Inheritance studies were conducted by using a single-plant selection (PP261-I) of PI 48226I and the ZYMV-susceptible watermelon cultivar New Hampshire Midget. In F1, F2, and reciprocal backcross populations, resistance was conferred by a single recessive gene to which the symbol zym is assigned. There was no linkage between zym and one of the three homodimer bands of the isozyme phosphoglucoisomerase (Pgi-2b), which was found in PP261-1, and in other ZYMV-FL resistant plants.
R. Provvidenti and R.O. Hampton
Resistance to white lupin mosaic virus (WLMV), a recently characterized member of the potyvirus group, was found in pea (Pisum sativum L.) plant introductions from Ethiopia (PI 193835) and India (PI 347485). In cross and backcross populations between plants of resistant PI 193835 with those of susceptible `Bonneville' and PP-492-5, this resistance was demonstrated to be governed by a single recessive gene. This gene was distinct from other genes previously found in PI 193835 and PP-492-5 (from PI 347492, India) conferring resistance to clover yellow vein virus (CYVV) and three strains of pea seedborne mosaic virus (PSbMV). Indirect evidence suggests that this newly recognized viral resistance gene, wlv, is a member of a cluster of closely linked genes located on chromosome 6. This gene cluster includes sbm-1, sbm-3, and sbm-4, which govern resistance to three PSbMV pathotypes, and cyv-2, which governs resistance to CYVV.
R. Provvidenti and David M. Tricoli
In a yellow summer squash (Cucurbita pepo L.) experimental line developed by Seminis Vegetable Seeds, the coat protein gene of an American strain of squash mosaic virus (SqMV-M88), conferred resistance to Arizona, California, New Jersey, and New York strains belonging to the two pathotypes of the virus. An analysis of genetic populations derived from crosses and reciprocal backcrosses of a homozygous SqMV-resistant line A127-1-2 with the susceptible cultivar Butterbar revealed that the high level of resistance mimics the response of a single recessive gene.
R. Provvidenti and C.L. Niblett
An Australian strain of passionfruit woodiness potyvirus (PWV-K) infected peas and caused a light to moderate mosaic consisting of chlorotic spots, veinal chlorosis, and some plant stunting. Resistance to PWV-K was found in pea cultivars and plant introductions (PI) known to be resistant to bean yellow mosaic virus (BYMV). In cross and backcross populations involving the resistant cultivar Bonneville (United States) and PI 140295 (Iran) with the susceptible cultivar Ranger, a single recessive gene was responsible for the high level of resistance to PWV-K. From crosses involving PI 391630 (China), which is resistant to BYMV but susceptible to PWV-K, and the cultivar Bonneville (resistant to both viruses), it was evident that different genetic factors conferred resistance to these two potyviruses. The symbol pwv is tentatively assigned to this newly recognized pea resistance gene. Three other Australian strains of PWV did not infect peas.