What causes brown rust disease in orchids?

OrchidsThe pathogen of brown rust disease is Uredo japonica. Symptoms: first, light brown or orange-brown small spots appear on the leaves, mainly on the middle and lower parts of the leaves, and in some cases the whole leaf is affected; afterward the spots gradually develop and become black patches (the lesions are irregular in shape), causing the whole leaf to turn yellow and finally to wither, dry out and die. The onset period of brown rust disease this year was from mid-April to early June. The main causes of brown rust disease are excessively wet potting soil and the effect of low spring temperatures. Between March and April this year, temperatures in southern China were lower than usual, with many rainy days and rainfall above normal, making it a rare low-temperature spring year since the founding of the People's Republic. Low temperatures and high humidity caused open-aircultivationoforchidsto develop brown rust disease, causing certain losses. The reasons are mainly as follows. First, extensive open-groundcultivationis the main cause inducing brown rust disease. Brown rust disease generally does not occur under conditions where the plants grow well or arecultivatedin a relatively dry state. From 1992 to 1995, this disease never occurred. This year, the average temperature in my area in March and April was 3-6 degrees lower than usual. The subjective cause of the outbreak of brown rust disease was a lack of awareness of protection, and objectively no frame shed covered with nylon film was set up and the plants were not moved indoors for cultivation. Any kind oforchidcultivated with shelter or under eaves that avoid rain did not develop the disease. Second, under unsuitable cultivation conditions, excessive flowering aggravates the development of the disease. This yearCymbidium goeringiiflowered late, about 10 days later than usual; some varieties did not bloom until March 5, and the flowering period was about 30 days, with many flowers, consuming a large amount of nutrients and affecting the disease resistance of the orchid plants. When the potting soil is too wet, root development is poor or hindered, making the plants highly susceptible to invasion and damage by pathogens. High humidity conditions favor the reproduction and spread of pathogens. Surveys found that orchid plants with more flowers and a longer flowering period were more severely affected than those with fewer flowers and a shorter flowering period. By orchid type, the severity of the disease was in the orderCymbidium goeringii、Cymbidium kanran, autumn orchids,Cymbidium faberi. BecauseCymbidium goeringiiencounters low temperature and high humidity during and after flowering, and consumes a large amount of nutrients during the flowering period, its disease resistance is relatively weak;Cymbidium kanranalthough it rests after flowering, the time is not long either; autumn orchids enter the growth period from dormancy;Cymbidium faberiis at the stage of flower scape formation and early flowering, full of vigor. Different types of orchids differ in disease severity, and there are also differences in physiology and disease resistance. Third, transplanting and repotting cause tissue damage, making orchid plants susceptible to disease. For orchid plants transplanted in spring and cultivated in the open air, brown rust disease occurred most severely; theLianbanorchids I potted during this period developed severe disease. The mature plants of Luoshan spring orchids transplanted in October last year also developed severe disease. Fourth, brown rust disease is difficult to control, and only by emphasizing prevention over treatment can the disease be effectively controlled. The author used copper sulfate, Jun Du Qing, boric acid, Shi Bing Jing (containing triadimefon, which also controls scale insects) and others, but the control effects were all poor. After brown rust disease occurs, it spreads and expands quickly, and control is difficult, but as long as extensive management is avoided by placing plants indoors or setting up frame sheds covered with nylon film, and keeping the potting soil slightly dry, the purpose of control can be achieved. As long as orchids are cultivated with rain shelter, the low temperatures in southern China in March and April generally will not cause freezing damage to relatively cold-tolerant orchid types such as Cymbidium goeringii. Properly controlling the number of flowers of Cymbidium goeringii andCymbidium kanranand shortening the flowering period help improve the disease resistance of the plants and reduce the chances of pathogen invasion. During the disease period, spray unaffected orchid plants with thiophanate-methyl or triadimefon for prevention. (Zhu Xiongguan)