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Attempts at breeding squash (Cucurbita pepo) resistant to powdery mildew (Sphaerotheca fuliginea and Erysiphe cichoracearum) have heretofore been unsuccessful due mainly to association of the resistance trait with reduction in yield. The goal of the present work was to determine if heterozygous hybrids expressing partial resistance could be potentially valuable for squash growers. One heterozygous hybrid each of zucchini, cocozelle, and vegetable-marrow squash was grown alongside a leading susceptible commercial hybrid cultivar of the same type, under standard field conditions. Although fungicides were applied at recommended rates every 6 to 13 days to suppress powdery mildew, the susceptible hybrids became heavily infested later in the season. The resistant hybrids were similar to their respective commercially available susceptible hybrids in fruit appearance and other horticulturally important traits. The resistant hybrids yielded at least as well as the commercial hybrids early in the season and outyielded them later in the season.

A melon (Cucumis melo L.) breeding line derived from PI 414723 is resistant to three potyviruses,watermelon mosaic virus (WMV), zucchini yellow mosaic virus (ZYMV), papaya ringspot virus (PRSV), and to powdery mildew (PM). The inheritance and linkage relationships of these four resistances were studied in a segregating F2 population and derived F3 families from a cross between cultivar Top Mark and the resistant breeding line. Dominant monogenic inheritance of all four resistances was observed. We report that line 414723-4S3, which was initially selected as a source of ZYMV and WMV resistance, is also a source of dominant monogenic resistances to PRSV and PM race 1. We also report on genetic linkage (significant departure from independent segregation, χ2 = 58.1, p≪ 0.0001) between resistance to WMV and ZYMV. The map distance between these loci was estimated to be 7.5 cm. The genes for resistance to PM and PRSV segregated independently from each other, and from ZYMV and WMV resistance.

The current powdery mildew (Sphaerotheca fuligninea) resistant cucumber varieties suffer from leaf chlorosis during autumn, winter and early spring cultivation in the Netherlands. Therefore screening was carried out for novel powdery mildew resistance genes. From 177 accessions, derived from different sources, 108 accessions proved to be partially resistant to S. fuliginea. Crosses were made with 53 resistant accessions to distinguish the presence of novel genes. It is likely that the accessions C. sativus 2145, C. sativus LV 41, PI 188807, Vladivostokij, White and Yellow 1 have one or more recessive powdery mildew resistance genes, different from powdery mildew resistance genes of the line NPI, which was used for variety breeding. Powdery mildew resistance tests with S. fuliginea give similar results in different regions of the world.

International audience; Inheritance of resistance of five melon lines to two strains of Sphaerotheca fuliginea belonging to races 1 (Sf1) and 2 (Sf2) and to one strain of Erysiphe cichoracearum (Ec) have been studied. ‘PMR 45’ possesses one dominant gene controlling only Sf1. ‘WMR 29’ has one dominant gene for resistance to Sf1 and another for Sf2 and these genes seem to be linked. In line ‘PMR 5’, one dominant gene (or a group of three closely linked genes) is involved in the control of the three strains with one complementary gene for Sf1 and another one for Ec. ‘PI 124112’ has one dominant gene or two closely linked loci controlling Sf1 and Sf2 and two complementary different genes controlling Ec. ‘Nantais Oblong’ has one dominant gene controlling only Ec. A nomenclature of the genes described is proposed.

Reciprocal crosses between 13 cultivars of Zinnia elegans Jacq. and one source of Z. angustifolia HBK resulted in interspecific hybrids. Florets examined 14 days after pollination showed that all 13 cross combinations of Z. angustifolia x Z. elegans yielded embryos in 47.2% of the florets pollinated. Six cross combinations of Z. elegans x Z. angustifolia produced embryos in 4.6% of the total Z. elegans florets pollinated. Interspecific crosses resulted in seed-propagated hybrids when Z. angustifolia was the maternal parent. Hybrids were obtained from reciprocal crosses between the two species when embryos were excised and cultured. Morphologically, most hybrids resembled Z. angustifolia but were sterile. Colchicine treatments of selected sterile hybrids resulted in shoots producing fertile inflorescences. Preliminary tests showed that hybrids were resistant to powdery mildew (Erysiphe cichoracearum DC ex Mecat).

Sterile interspecific hybrids and colchicine-induced amphiploids of Zinnia elegansJacq. and Z. angustifolia HBK were examined to determine the mode of inheritance of resistance to Erysiphe cichoracearum DC ex Merat. Fertility was restored through colchicine treatment of two sterile hybrids of species reciprocal parentage which differed in ray petal response to the pathogen. Derived amphiploids were subsequently intercrossed to overcome the lack of segregation for this trait due to genetic control of pairing upon chromosome doubling. Resistance to E. cichoracearum appears to be complexly inherited in both leaves and ray florets of sterile hybrids and induced amphiploids. Two major dominant genes have been implicated in conferring resistance in ray petal tissue of derived amphiploids. Data obtained from the F1 hybrid progeny of the intercrossed amphiploids indicate that this trait is not cytoplasmically inherited. It is speculated that the genes conferring resistance in the ray florets are acting independently from those controlling leaf resistance and that most, if not all, of the resistance genes are inherited from Z. angustifolia.

Powdery mildew in cucumbers (Cucumis sativus L.) can be caused by two fungi, Erysiphe cichoracearum and Sphaerotheca fuliginea. In The Netherlands we have been mainly concerned with the latter, at least during the last few years.

The natural infection of Erysiphe cichoracearum on 29 accessions of five wild Lactuca species (L. serriola, L. saligna. L. virosa, L. aculeata and L. dentata) and on one hybrid of L. serriola x L. sativa has been investigated for two and three years, respectively. No infection was observed on L. serriola (PI 255665), L. saligna (LSA/92/1 and LSA/92/2), L. virosa (LVIR/26 and LVIR/57/1) and L. dentata (PI 234204). The level of susceptibility of the control cultivars of L. sativa was found to be moderate (‘Fila’) to fairly high (‘Hilde’).