Downy mildew infection process
After inoculation with Peronospora sparsa, the symptoms of rose leaves were shown in Fig. 1A. Compared with the control group, faded green, irregularly shaped patches appeared on the surface of the leaves 12 h after inoculation with P. sparsa, and the back of the leaves was covered with gray and white mold layers. The rear of the leaf develops a tiny amount of mycelium, the back of the leaf develops a small amount of conidial peduncle, the top of the leaf develops a pale yellow circular conidial sporangium, and one stoma develops a conidial peduncle that is sharply bifurcated. Angled branches that are dispersed all over the leaf vein (Fig. 1B). The patches gradually grew larger and turned yellowish brown 24 h after the vaccination. A significant amount of mycelium developed on the underside of the leaves, and conidial stem numbers and branches increased throughout time, along with conidial sporangium and clusters of 2–3 conidial stems within a stoma. The quantity of conidial peduncle and conidial sporangium rapidly grew along with the number of mycelia. The nutrients were sufficient at this time, and the conidial spores were mainly elliptical in shape. When the pathogen enters the host cell via the haustorium to draw up nutrients, the epidermal cells of the leaves are damaged, and the vein system begins to deteriorate (Fig. 1C). The lesion turned grayish brown 36 h after inoculation, with a nearly round brown burning lesion in the center, the mildew layer on the back of the leaf turned dark and gray, the mature coniferous sporangium began to fall off, and the P. sparsa ingested a lot of nutrients from the leaf, destroying the cell structure. The more nutrients the pathogenic bacteria consumed, the more epidermal cells ruptured and perished, and the palisade tissue was severely damaged (Fig. 1D). The burning lesions were 2–6 mm in diameter after 48 h of inoculation, with uneven margins and few minor polygonal shapes confined by leaf veins. At this time, the infected leaves easily fall off the plant with a touch of the finger or a breeze, and the branches of the serious season become bare sticks, the leaves yellow and wilt, and the fresh tips die of corruption. Many mature conidiangium came off and landed on the surface of the leaves or in the soil. Under the electron microscope, the inclusion of mesophyll cells could not be observed; only the contour of the cells was uneven in shape and chaotic; it was difficult to distinguish palisade tissue and sponge tissue, and a large number of leaf vein cells died.
Field investigation on downy mildew resistance of different cut rose varieties
The results of a field experiment with twenty-six cut rose types in the Jinning district are shown in Table 1. The disease index for each type was calculated based on its resistance to illness. The results contained three high resistant types, five resistant varieties, six medium resistant varieties, three moderately susceptible varieties, five susceptible varieties, and four highly susceptibility variants. The gloss and texture of the leaves may influence their disease resistance. According to Table 1, the leaves of high-resistance types were shiny, stiff, and thicker, whereas the leaves of high-sensitivity variants were dull, soft, and thinner.
Table 1
The resistance of various cut rose types to downy mildew.
Number | Species | Resistance | Leaf glossiness | leaf texture |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | R hybrida ‘Chizizhixin’ R hybrida ‘Gaoyuanhong’ R hybrida ‘Doushiliren’ R hybrida ‘Mizhi’ R hybrida ‘Miyue’ R hybrida ‘Feicui’ R hybrida ‘Xiaoqiao’ R hybrida ‘Cappuccino’ R hybrida ‘Chuanqi’ R hybrida ‘Pink Floyd’ R hybrida ‘Hongchuan’ R hybrida ‘Menta’ R hybrida ‘Quick Sand’ R hybrida ‘Weiguang’ R hybrida ‘Maoyao’ R hybrida ‘White O'Hara’ R hybrida ‘Free Spirit’ R hybrida ‘Creamy Reeva’ R hybrida ‘Creamy Reeva’ R hybrida ‘Jiaoao’ R hybrida ‘Haiyangzhixing’ R hybrida ‘Jumilia’ R hybrida ‘Mandala’ R hybrida ‘Nightingale’ R hybrida ‘Peach Avalanche’ R hybrida ‘P Sweet Avalanche’ | HR HR HR R R R R R MR MR MR MR MR MR MS MS MS S S S S S HS HS HS HS | Yes Yes Yes No No No Yes No No No No No No Yes No No No Yes No No Yes No No No No No | Thick Thick Thick Thick Thick Thick Thick Thick Thick Thick Thick Thick Thick Thick Thick Thick Thin Thick Thin Thin Thin Thin Thin Thin Thin Thin |
Leaf symptoms of cut rose after inoculation with P. sparsa
The leaf symptom of high resistant variety R hybrida ‘Chizizhixin’ and high susceptibility variety R hybrida ‘Nightingale’ after inoculation with P. sparsa were varied (Fig. 2E-F). Yellow-green spots occurred on the leaf surface of R hybrida ‘Nightingale’ after 12 h inoculation with P. sparsa, whereas no disease spots appeared on the leaf surface of R hybrida ‘Chizizhixin’. The region of R hybrida ‘Nightingale’ leaf lesion continued to increase after 24 h of inoculation. After 36 h of inoculation, yellow spots appeared on the leaf surface of R hybrida ‘Nightingale’. After 48 h of inoculation, the whole leaves of R hybrida ‘Nightingale’ turned yellow, the leaflets fell off, and a tiny number of yellow disease spots appeared on R hybrida ‘Chizizhixin’. It can be noted that the onset period of R hybrida ‘Chizizhixin’ is later than that of R hybrida ‘Nightingale’ and the incidence degree is lower.
Microscopic observation of cut rose inoculated with P. sparsa
The electron microscope and scanning electron microscope observation of the lower epidermis of the leaves of two cut rose varieties inoculated with P. sparsa were shown in Fig. 3. The stratum corneum cells on the back of the leaves of R hybrida ‘Chizizhixin’ were apparent and closely grouped after inoculation at 0 h, and the stomata were small and the opening was small. The stratum corneum cells on the back of R hybrida ‘Nightingale’ leaf are fuzzy, with big and open pores (Fig. 3I-J). The spores on the back of the leaves of R hybrida ‘Nightingale’ germinated 12 h after inoculation, a small number of conidial pedunculi emerged from the stomata, and a pale yellow circular conidial sporangium grew from the tip. The spores of the leaf back of R hybrida ‘Chizizhixin’ germinated after inoculation for 24 h. The number of spore germination of R hybrida ‘Nightingale’ continued to increase. The conidial sporangium of R hybrida ‘Nightingale’ began to slip off the conidial stalk 36 h after inoculation, but the quantity of spores germinating from R hybrida ‘Chizizhixin’ increased (Fig. 3G-H). The conidial sporangium of R hybrida ‘Nightingale’ flaked off 48 h after inoculation. The conidial sporangium of R hybrida ‘Chizizhixin’ began to break away from the conidial stem.
Determination of defense enzyme activity in cut rose infected with P. sparsa
By comparison, we found that a series of changes of five defense enzymes occurred between the highly resistant variety R hybrida ‘Chizizhixin’ and the sensitive variety R hybrida ‘Nightingale’ when infected by pathogenic bacteria (Fig. 4). The activity of MDA and CAT both showed the similar changing trend, with both first increasing and subsequently declining. The MDA content of R hybrida ‘Nightingale’ began to peak at 12h after inoculation, while R hybrida ‘Chizizhixin’ had the highest peak at 24 h after inoculation. The activity of POD and PPO demonstrated a constant changing trend, with both showing a steady increasing trend. At 0h inoculation, the POD activity and PPO activity of R hybrida ‘Chizizhixin’ were significantly higher than that of R hybrida ‘Nightingale’, and at 12 h after inoculation, R hybrida ‘Chizizhixin’ had significantly higher POD and PPO activity, while R hybrida 'Nightingale' had little. at 0h, 12 h, 24 h, 36 h, and 48 h, SOD activity showed a trend of continuous decline, with R hybrida ‘Chizizhixin’ having a higher SOD content than R hybrida ‘Nightingale’.