Helena Eyal

Ultrastructure, autofluorescence, callose deposition and lignification in susceptible and resistant muskmelon leaves infected with the powdery mildew fungus Sphaerotheca fuliginea

Abstract Leaves of the susceptible cv. Ananas-Yokneam (AY) and of resistant cvs PI 124111F, PI 124112, PMR-45, PMR-6 of muskmelon ( Cucumis melo ) were inoculated with either race 1 or race 2 of Sphaerotheca fuliginea and examined microscopically after staining with Calcofluor, basic aniline blue or phloroglucinol. At 20–24 h post inoculation of the susceptible AY at 23 °C, the fungal spores (both race 1 or race 2) developed one or two germtubes which penetrated into one or two epidermal cells. The penetration zones were surrounded with callose-like material but no autofluorescence nor lignin-like materials were observed in the penetrated epidermal cells. A similar response was observed in PMR-45 inoculated with race 2 (compatible). In contrast, the fungus developed a single germtube on the resistant PI 124111F, PI 124112, and PMR-6 inoculated with either race, as well as in PMR-45 inoculated with race 1, which induced autofluorescence, callose accumulation and lignification in the penetrated epidermal cells. Electron microscopical studies revealed that the rapid collapse of epidermal cells in the resistant cultivars was accompanied by the accumulation of callose-like deposits in cell walls and around haustoria, electron-opaque deposits in the plasma membrane and electron-opaque deposits between the cell wall and the plasma membrane. Occasionally, callose also appeared in epidermal and mesophyll cells adjacent to the penetrated cells. By 96–120 h post inoculation, abundant sporulation was observed in the compatible interactions whereas only 1–3 germtubes with no sporulation were seen in the incompatible interactions. Heat shock or chemical inhibitors (cycloheximide, blasticidin-S, cordycepin, 2-deoxy-D-glucose, α-aminooxyacetic acid, 2,4-dinitrophenol and sodium azide) failed to induce susceptibility in the resistant cultivars. The results suggest a similar structural response to powdery mildew in C. melo cultivars carrying the resistance genes Pm-1 (PMR-45), Pm-2 (PMR-6), Pm-3 (PI 124111F), Pm-4 (PI 124112), Pm-5 (PI 124112) and Pm-6 (PI 124111F).

Ultrastructure of Pseudoperonospora cubensis in muskmelon genotypes susceptible and resistant to downy mildew

Abstract The causal agent of muskmelon (Cucumis melo L.) downy mildew, Pseudoperonospora cubensis (Berk. et Curt.) Rost., produced 10–15-mm lesions in the susceptible cv. Ananas-Yokneam (AY) but 1–3-mm, water-soaked lesions in the resistant cv. PI 124111F (PI). Sporulation on cv. PI was extremely limited compared to the conspicuous sporulation on cv. AY. In both cvs the fungus produced intercellular hyphae, rich in β-1,3-glucans, from which digitate (cauliflower-like) haustoria grew into the host cells. The haustoria had induced only minor chemical and ultrastructural changes in the cytoplasm and/or walls of cv AY 144 h after inoculation, whereas in contrast major changes were observed in cells of cv. PI as early as 20 h after inoculation. Changes induced in cv. PI included a heavy deposition of paramural, layered, callose-like materials along the inner surfaces of the host cell walls; enrichment of host cell walls with lignin-like materials (staining with phloroglucinol and chlorine sulphite); and encasement of the haustoria with heavy deposits of callose-like materials staining with resorcinol blue and lacmoid, fluorescing with aniline blue, and from TEM observations). In addition, the intercellular hyphae in cv. PI, but not in AY, sometimes produced haustoria (and necrosis) in epidermal cells, which gave rise to the early appearance (72 h) of water-soaked lesions. It is proposed that the containment of host cells and haustoria by callose- and lignin-like materials may interrupt the flow of nutrients from and into the invaded host cells.

Downy mildew-, powdery mildew- and fusarium wilt-resistant muskmelon breeding line PI-124111F

A muskmelon (Cucumis melo L.) breeding line, PI-124111F, is a seventh-generation selection derived from PI-124111, which is resistant to downy mildew (Pseudoperonospora cubenis pathotype 3), powdery mildew (races 1 and 2 ofSphaerotheca fuliginea) and Fusarium wilt (Fusarium oxysporum f. sp.melonis, races 0, 1 and 2). This is the only breeding line known to carry multiple-race resistance to these diseases. PI-12411IF is a monoecious muskmelon with poor fruit characteristics.

Growth and differentiation of sporangia and sporangiophores of Psudoperonospora cubensis on cucumber cotyledons under various combinations of light and temperature

Abstract The inhibitory effect of light upon sporangial and sporangiophore formation of Pseudoperonospora cubensis on cucumber cotyledons was studied. The following major findings may be pointed out. (a) Light, especially of the blue region of the spectrum, inhibited sporangial production but did not prevent emergence of sporangiophores through stomata. Sporangiophores emerging from leavesexposed to high light level were abnormal, but could further produces sporangia upon transfer to darkness. (b) The inhibitory effect of light upon sporangial formation was temperature dependent. The average rates of inhibition, as compared to dark control, were 18, 84 and 90% at about 15, 20 and 24 °C, respectively. (c) At 10 °C, light did not inhibit, but rather stimulated sporangial production. (d) Preceding dark treatments (PD) of 2 to 4 h greatly diminished the inhibitory effect of light—more so at 20 to 30 °C than at 10 °C, but inhibition was fully restored upon extending the PDT to 6 h. (e) Indirect irradiation, achieved by partial covering of a leaf with an aluminum planchette, reduced sporangial yield of the pathogen on the unirradiated tissue. The possible involvement of a chemical and/or physical antisporulant in the inhibitory effect of light is discussed.

Differential expression of resistance to powdery mildew incited by race 1 or 2 ofsphaerotheca fuliginea inCucumis melo genotypes at various stages of plant development

Thirty-nine genotypes ofCucumis melo (plant introduction entries, open-pollinated cultivars and F1 hybrids) were evaluated for resistance to powdery mildew under either natural field conditions or artificial inoculation in growth chambers at the cotyledonary stage and the 2-true-leaf stage. Results confirmed that susceptibility in cotyledons was not necessarily associated with susceptibility in either true leaves in growth chambers or adult plants in the field. However, resistance at the 2-true-leaf stage in growth chambers was highly correlated with resistance of field-grown plants. Results also showed that 20 muskmelon genotypes resistant to race 1 at the cotyledonary stage were also resistant at the 2-leaf-stage and as adult plants in the field. The same was true for ten genotypes with race 2 inoculations. Because muskmelon genotypes expressing resistance in cotyledons were also resistant in true leaves in growth chambers or the field, the use of plants at the cotyledonary stage is recommended for screening for powdery mildew resistance caused by race 1 or race 2 ofS. fuliginea. When cotyledons are susceptible, screening should be done at the 2-true-leaf stage.

A preformed chemical inhibitor of tobacco powdery mildew on leaves of Nicotiana glutinosa

A diterpene identified as 13(s)-hydroxylabda-8(20),14-dien-2-one (2-ketoepimanool) (C20H32O2) was isolated from epicuticular leaf “waxes” of Nicotiana glutinosa, a species immune to powdery mildew of tobacco. When externally applied to leaf surfaces of susceptible tobacco plants this diterpene strongly suppressed the emergence of conidial germ tubes 2 and 3 (but not of germ tube 1) and hence inhibited mildew development (ED90 = 1 μg cm−2). The compound was not detected in N. debneyii or N. tomentosiformis, two other species immune to mildew, nor in three immune cultivars of N. tabacum.

Effects of light during infection on the incidence of downy mildew (Pseudoperonospora cubensis) on cucumbers

Abstract Pseudoperonospora cubensis , the causal agent of downy mildew of cucurbits, infected cucumbers under either dark or light conditions. Infection at 20 °C was markedly enhanced when a short wet period was associated with illumination of relatively low photon flux density. The optimal temperature for infection in darkness at 2 and 4 h of wet period was 25 and 15 °C, respectively, whereas in light it remained constantly at 20 °C. About 90% of the zoospores of the pathogen settled on stomata of illuminated cucumber leaves within 2 h from inoculation with freshly-prepared sporangial suspension compared to about 75% that did so within 3 h in darkness. Fungal penetration occurred through stomatal openings. Darkness and DCMU did not prevent zoospore settlement on stomata, hence, high disease levels were obtained on DCMU-treated plants inoculated in darkness. When wetness was applied for a short period of 2·5 h, plants exhibited some diurnal periodicity in proneness to infection which was highest at midday. The results explain the high incidence of downy mildew in dry seasons in semi-arid growing areas where dew is the major source of free leaf moisture.

Host-induced sensitivity of corn rust uredospores to dryness

Abstract Exposure of corn plants inoculated with Puccinia sorghi to intermittent wetting during infection reduced subsequent disease development. Greatest inhibition of disease development was observed in plants wetted for 2 h before exposure to dryness. Microscopical examination revealed that in such plants uredospore germination and germ tube growth were greatly inhibited. Uredospores exposed to intermittent wetting on water agar or on Millipore membranes retained a high degree of germinability and infectivity to corn plants. It is suggested that sensitivity of corn rust uredospores to desiccation is host-induced.