لخّصلي

Xoo enters the rice leaf typically through hydathodes at the leaf tip and leaf margin. Cells on the leaf surface may become suspended in guttation fluid as it exudes at night and enter the plant by swimming, or passively as the fluid is withdrawn into the leaf in the morning Bacteria multiply in the intercellular spaces of the underlying epitheme, then enter and spread into the plant through the xylem Xoo may also gain access to the xylem through wounds or openings caused by emerging roots at the base of the leaf sheath. Within the xylem, Xoo presumably interacts with xylem parenchyma cells . The pathogen moves vertically through the leaf through primary veins but also progresses laterally through commissural veins. Within a few days bacterial cells and EPS fill the xylem vessels and ooze out from hydathodes, forming beads or strands of exudate on the leaf surface, a characteristic sign of the disease and a source of secondary inoculum. Xoc, by contrast, penetrates the leaf mainly through stomata, multiplies in the substomatal cavity and then colonizes the intercellular spaces of the parenchyma. Like Xoo, Xoc may also gain access through wounds, but it remains restricted to the apoplast of the mesophyll tissue and does not invade the xylem . Xoc also exudes from natural openings in the leaf in chains or strands, or under moist conditions as small beads of ooze. Yellow exudate on the leaf surface is a typical sign of BLS, and as is the case for Xoo, it may fall into irrigation water or be dispersed by wind, rain, insects or other means, and contribute to spread of the disease
BB and BLS symptoms are easily differentiated in the early stages of disease and reflect the different modes of infection by each pathogen. Foliar symptoms of BB usually become evident at the tillering stage as small, green water-soaked spots at the tips and margins of fully developed leaves. The spots expand along the veins, merge, and become chlorotic and then necrotic, forming opaque, white to grey coloured lesions that typically extend from the leaf tip down along the leaf veins and margins. BLS symptoms, by contrast, begin with small, water-soaked lesions anywhere along the leaf between the veins. Veins act as barriers as infected areas expand and coalesce lengthwise, resulting in the symptom for which the disease is named. Streaks are translucent and typically yellow . At later stages infected leaves turn greyish white and die. When infection results from entry through breaks in the leaf as might occur due to high wind, symptoms may extend across the leaf break and expand lengthwise killing most or all of the leaf. As both diseases progress their symptoms become less differentiable. Leaf blades may wilt and roll as they dry up and die. Leaves with BLS may also turn white or grey from the growth of opportunistic or saprophytic fungi, and thus resemble BB. Depending on the growth conditions or the degree of resistance of the cultivars, BB and BLS may be confused with each other or with unrelated physiological disorders of the plant. Xoo and Xoc often occur in rice fields simultaneously, and individual leaves may show symptoms of both diseases (R. Sonti, personal communication).
In the tropics, and particularly on susceptible cultivars of O. sativa ssp. indica, Xoo causes two disease syndromes with symptoms distinct from typical bacterial blight: kresek and pale-yellow leaf . Kresek is a seedling blight that occurs shortly after transplant from nurseries to the field. The common practice of cutting leaf tips before transplanting plays an important role in the development of the syndrome. Cut leaves serve as an infection court for the pathogen, and after a few days, water-soaked spots develop just beneath the cut tips. In addition, broken roots resulting from pulling seedlings off the seedbed serve as entry points for bacteria present in flood-irrigated fields. Bacteria spread through the vascular system to the growing point of the plant, infecting the base of other leaves, and killing entire plants in 2–3 weeks. Plants that survive kresek suffer arrested tiller growth, a stunted appearance and an overall yellowish-green colour. Pale-yellow leaf is observed in older plants and is sometimes considered a secondary effect of seedling leaf blight and wilt. Whereas older leaves appear green and healthy, younger leaves are uniformly pale yellow or whitish, and tillers do not grow fully.