What to Do When a Phalaenopsis Orchid Gets Gray Mold? Symptoms and Control Methods
Gray mold is one of the most common diseases in Phalaenopsis orchid production, occurring frequently during their growing season, especially during winter and spring greenhouse cultivation, and the pathogen can infect any part of the plant. If cultivation management is neglected, it further favors the occurrence and spread of the disease, and in severe cases can cause a large number of fallen flowers and leaves, not only affecting the plant's blooming but also directly reducing the ornamental value and yield of Phalaenopsis orchids.
(1) Conditions for Onset
Warmth and humidity are the main conditions for the spread of gray mold in Phalaenopsis orchids. That is, the disease is most likely to occur when relative humidity is around 90% and temperature is between 18 and 25°C. When air humidity is high, the disease develops rapidly; conversely, the disease develops slowly with less gray mold; when soft pots are placed too densely, or drainage is poor, it occurs severely in greenhouses with poor ventilation; when nitrogen fertilizer is excessive and plant tissues are tender and weak, the disease is severe. After consecutive cloudy days with low temperatures, when the weather clears and temperatures rise, gray mold is prone to outbreak. When the disease occurs, mature conidia spread through air currents, irrigation water, rainwater, greenhouse drips and field operations. The pathogen has strong pathogenicity to flowers and leaves. Healthy and vigorous plants are less susceptible to infection.
(2) Damage Symptoms
The disease mainly damages the leaves and flowers of Phalaenopsis orchids. When damaging leaves, it mainly occurs on the underside of leaves, appearing as brown patches distributed unevenly, affecting leaf glossiness; about 24 hours after the flower parts are damaged, semi-transparent round small spots appear scattered on the petals and sepals, appearing water-soaked, and later the spots turn brown to dark brown with pink edges, and from flower damage to shedding, the number of spots gradually increases. On white series flowers, the spots are more noticeable, and in severe cases flowers wither prematurely and buds dry up. Because its flowering period lasts 2 to 3 months, the chance of infection increases accordingly, especially for flowers that bloom earlier, which are more susceptible to this disease.
(3) Control Methods
1. Reduce Infection Sources: At any time, remove diseased flowers, diseased leaves, withered flowers and weeds inside and outside the garden and other residues.
2. Strengthen Field Management (1) Pay attention to controlling the amount of nitrogen fertilizer to prevent excessive vegetative growth and worsen the disease, so as to keep the orchid plants vigorous and improve disease resistance. (2) In the early stage of disease, promptly remove the diseased parts and burn them in a centralized manner. (3) Note that sufficient space should be left between soft pots to allow ventilation and light penetration. (4) Water according to "see dry, see wet," and prevent water accumulation. (5) During management, minimize plant damage as much as possible to prevent pathogens from entering through wounds. (6) Ventilate in a timely and appropriate amount to avoid cold damage and heat damage.
3. Chemical Control (1) For prevention, spray with 70% thiophanate-methyl wettable powder at 2500 times dilution, or 50% prometryn wettable powder at 2000 times dilution. (2) In the early stage of disease, spray with 50% procymidone wettable powder at 2000 times dilution, or 65% dimethachlon wettable powder at 1500 times dilution for control. (3) In the early flowering stage, spray with 50% prometryn wettable powder at 2000 times dilution, or 50% iprodione wettable powder at 1500 times dilution, spraying once every 2 to 4 weeks for prevention. When the disease occurs, use 1000 times dilution of vinclozolin, or 2000 times dilution of procymidone, spraying once every 7 days for control. (4) To avoid gray mold developing resistance to fungicides, effective agents should be applied in rotation.