Many trials have demonstrated that apple and pear trees treated with the plant growth regulator prohexadione-Ca (BAS 125 W) are less susceptible to infestation by the bacterial disease fire blight. In further investigations we have studied the effect of this compound against fungal diseases, concentrating on scab (Venturia inaequalis) in apple. Working with apple seedlings and artificial inoculation under greenhouse and field-like conditions, scab infestation could be reduced by applications of prohexadione-Ca. Whereas this effect was rather marginal if inoculations were made shortly after treatment, highly significant effects were found in the time span of ≈1 to 4 weeks after application. Preliminary results from trials conducted under orchard conditions support these findings. We assume that, similar to the situation with fire blight, changes in phenylpropanoid metabolism are mainly responsible for the reduced scab incidence. It should not be ruled out, however, that anatomical and morphological changes caused by prohexadione-Ca may also contribute to this effect.
The presence of sporulating lesions on seedlings inoculated with the apple scab fungus in the greenhouse was the best criterion for discarding susceptible individuals, and provided a means of selecting other factors for resistance in addition to the Vf gene.
Short photoperiods (8 hr) and low light intensities (1000 ft-c) significantly reduced the percentage of resistant seedlings. This emphasized the importance of growing the seedlings under uniform conditions.
Progenies from crosses of resistant by susceptible parents (Vfvf × vfvf) were grown under various controlled regimes of light, temperature, and humidity. Inoculation with the scab fungus resulted in significantly different percentages of resistant seedlings, indicating genetic differences in resistance between these progenies. When inoculation results of progenies of susceptible varieties were summarized, the percentages of resistant seedlings varied with the observed susceptibilities of the parents. These genetic differences should be taken into consideration in a breeding program.
The potential impact of propane flamers on the development and release of ascosporic inoculum of Venturia inaequalis (Cke.) Wint. from infected dead apple (Malus domestica Borkh.) leaves that overwintered on the soil of an experimental orchard was assessed. Thermal reduction of scab primary inoculum was first conducted under controlled conditions using an indoor testing facility. At the time of ascospore maturation, heavily infected leaves were submitted to temperature rises ranging from 150 to 200 °C with open-flame burners, thus reducing the number of ascospores subsequently released by 76% and 87%, respectively. During Spring 1995, thermal treatments of overwintered dead leaves were performed directly on the ground of an apple orchard with an experimental propane flamer design to generate uniform heat at ground level. Four thermal treatment strategies, involving two dates of flaming and two heat intensities, were tested. Flaming orchard ground, when performed in early May, before significant development of ground cover, reduced the number of ascopores released from infected dead leaves by half. A significant residual effect of the treatments on ascospore ejection was still observed 2 and 4 weeks after the treatments, thus indicating that ascospore maturation inside the leaves may be reduced by heat treatment. These results indicate potential for propane flamers to reduce apple scab primary inoculum in orchards.
`Ariane' is a new apple cultivar carrying the Vf gene of resistance to Venturia inaequalis. It was derived from a cross made in 1979 at INRA in Angers, France between two advanced selections. The seed parent was a hybrid between the Vf cultivars `Florina' and `Prima' and the pollen parent was derived from an open pollination of `Golden Delicious'. `Ariane' matures during the same period as `Golden Delicious'. `Ariane' fruit has blushed bright red skin, with 75% of the fruit surface colored. It is medium sized. The flesh is firm and crisp; it has excellent flavor and eating quality with a high level of sugar and acid. Storage ability is long. Trees have to be thinned intensively, chemically and manually, to get good size fruit. `Ariane' shows low susceptibility to powdery mildew and fire blight. `Ariane' has been selected by the SARL (Ltd) NOVADI and the S.A.S. POMALIA to be developed as the first cultivar of a “Les Naturianes” series. Since 2002, about 95,000 trees of `Ariane' have been planted in France.
Vegetable oil emulsion (VOE) was applied to `Gala' and `Fuji' apple (Malus ×domestica) trees after harvest to hasten defoliation and reduce apple scab (Venturia inaequalis). Applied at 2%, 4%, or 6%, VOE applied to whole trees in the fall induced leaf drop, with the highest concentration causing the most defoliation. At the same concentration, VOE applied in early, mid, or late October had similar effects on leaf drop. VOE treatment reduced respiration and stimulated ethylene production in shoot tissue within 24 hours of application. None of the treatments affected tree hardiness during the winter, or shoot growth the following spring. Return bloom density was unaffected; however, VOE tended to delay anthesis by 2 to 5 days. Under controlled conditions, `Gala' and `Fuji' trees inoculated with scab spores developed 48% and 65% scab, respectively. VOE-induced defoliation reduced scab by 50% to 65%. VOE-induced defoliation plus manual leaf removal from the orchard floor, or VOE-induced defoliation in late fall (15 Oct.-15 Nov.) plus application of 5% lime sulfur in early spring, controlled scab to <5% on both leaves and fruit. Neither lime sulfur nor urea applied in late fall at 2% induced defoliation or controlled scab. VOE at 4% plus 2% lime sulfur and/or 2% urea applied in late fall, however, defoliated `Gala' trees effectively and controlled scab on fruit to <7% the following spring. In the `Fuji' planting, the combination of 4% VOE plus 2% lime sulfur and 2% urea reduced scab on fruit from 21% in controls to 0%.
The genetic basis of resistance to apple scab [Venturia inaequalis (Cke.) Wint.] in the Russian apple seedling R12740-7A (Malus Mill. sp.) was investigated. Segregation ratios obtained in crosses with susceptible cultivars suggested that at least two genes were involved, and three foliar resistance reactions (chlorotic, stellate necrotic, and pit type) were observed after inoculation. DNA markers were identified for both the stellate necrotic (Vr ) and pit type (no locus designation, Vx suggested) resistance phenotypes. Comparison of resistance phenotypes with marker segregation demonstrated that only two major dominant genes were present in R12740-7A, one producing the stellate necrotic lesion and the other the pit-type lesion. The chlorotic lesion could be attributed to either unclear expression of the resistance phenotype or to susceptible genotypes not contracting the disease. These markers along with a previously published marker for Vf were used to analyze inheritance of resistance in a Vr × Vf cross in advanced breeding material. The markers identified successfully all susceptible progeny, as well as apparent escapes and individuals possessing both Vf and Vr . Thus, the markers should be useful in future screening of segregating progeny and in the pyramiding of scab resistance genes in new cultivars.
resistance to apple scab caused by the fungal pathogen Venturia inaequalis (Cke). In scab-susceptible crabapples, symptoms of infection include defoliation by early summer coupled with loss of winter hardiness and even death resulting from repeated
collection locations. Disease resistance. Potted seedling plants were inoculated with conidia of mixed races (1–5) of Venturia inaequalis for two consecutive years. Seedlings scored as resistant had the following response: no symptoms, pin
.L. Rikkerink, E.H.A. Gardiner, S.E. Bassett, H.C.M. Kodde, L.P. Plummer, K.M. 2005 The Vh8 locus of a new gene-for-gene interaction between Venturia inaequalis and the wild apple Malus sieversii is closely linked to
, and Venturia inaequalis were subcultured and analyzed on Potato Dextrose Agar (PDA) (Becton, Dickinson & Co., Sparks, MD) prepared according to the manufacturer's specifications. Cultures of Phytophthora infestans , Phytophthora megasperma