Why do new orchid seedlings rot at the core? Causes and prevention methods
Core rot in new orchid seedlings is caused by black rot, with the pathogen being Pythium. Once core rot is discovered in new orchid seedlings, there is no cure, which deeply saddens orchid lovers. Prevention is the key to core rot. After several years of observation and practice, the author shares some insights for reference only.
(1) Water damage: The first rain after a long drought is mostly acid rain carrying dust pollutants; heavy downpours during the hot season leave orchid seedlings soaked in water for extended periods; continuous drizzle in spring and autumn keeps orchid seedlings in a high-humidity state for long periods. When new leaves of orchid seedlings are waterlogged by acid rain, heavy rain, or drizzle, core rot can result. After new buds open their leaves, improper watering methods - such as watering directly from above the new leaves - can cause water to linger too long in the leaf core, also possibly leading to core rot. Therefore, attention should be paid to rain protection, and soaking the pot or watering from the side of new seedlings should be adopted as much as possible to prevent water from pooling in the leaf core of new seedlings.
(2) Pest damage: Pests such as scale insects, red spider mites, thrips, aphids, slugs, and snails prefer to suck the leaf sap of new seedlings. After biting and wounding the leaf core, the leaf core of new seedlings is highly susceptible to Pythium infection, leading to core rot. Regular preventive spraying with insecticides and fungicides such as parathion, omethoate, copper hydroxide, thiophanate-methyl, and carbendazim can be used.
(3) Chemical and fertilizer damage: Excessive use of hormones and overdosing of chemical fertilizers. Fertilizers such as Orchid King cause the nitrogen, phosphorus, and potassium needed for orchid growth to become imbalanced, with a lack of trace elements, causing new seedlings to grow excessively fast with soft, floppy leaf stems. In mild cases, the basal sheaths of leaves are scorched; in severe cases, tissue necrosis occurs at the leaf core of new seedlings, turning brown and black. Therefore, avoid greed and never seek quick results; apply chemicals appropriately according to standard requirements, and use dilute fertilizer applied frequently.
(4) Scalding: The leaf core of new seedlings is very tender. If the temperature difference between the water used for watering orchids and the leaf surface temperature of the orchids is too large - when the temperature difference exceeds 10 degrees - it can easily scald the leaf core of new seedlings, which is then eroded by Pythium, causing core rot. Therefore, the water used for watering orchids should be left standing in the orchid garden for a period so that the water temperature approaches the orchid leaf temperature before watering; during hot seasons, watering should be done in the early morning or evening as much as possible, and never pour tap water directly on the plants during the high temperatures of the afternoon.
(5) Poor ventilation: Hot seasons with high temperatures and humidity are the peak period for core rot. When the orchid garden has high temperature and high humidity, if ventilation is poor and the base of new seedlings lacks airflow, core rot is easily triggered. Therefore, when the temperature in the orchid garden exceeds 30 degrees, special attention should be paid to management. The orchid garden should be shaded to cool down, maintaining air circulation inside and out, with humidity controlled at around 75%: the growing medium should be loose and breathable, and the base of new seedlings must not be buried too deep - it is best to leave the pseudobulb area of new seedlings slightly exposed.