Causes and Solutions for Orchid Leaves Turning Yellow and Black, Root Rot, and Death in Summer

Of all the seasons, orchids are hardest to grow in summer. All kinds of pests and diseases occur frequently, and orchid plants either develop yellow leaves or root rot, which is endlessly frustrating. Today I will teach you 4 tricks to solve the problem of orchids easily rotting in summer!

The most common problem for orchids in summer is disease. Pests are relatively easy to deal with. Most things that threaten an orchid's life are diseases; pests can only assist diseases in threatening an orchid's life. The main common diseases in summer are: anthracnose, stem rot, southern blight, and soft rot. These 4. Let us get to know them one by one!

Before explaining these 4 diseases, let us first clarify one issue: most orchid diseases show up on the leaves. By observing orchid leaves with our eyes, we can tell when it is disease and when it is not. Now I will teach you a simple little trick to easily figure it out. If the leaf yellows from the top downward, it is generally a normal physiological phenomenon or a normal physiological disorder, so do not worry and do not fuss over it; if the leaf yellows from the bottom upward, you must pay attention and promptly find a way to treat it. You cannot delay, because a fast-developing case can kill the entire orchid plant in just a few days! Today we will not discuss physiological disorders, only the 4 common diseases that are seriously harmful to orchids!

First: anthracnose. Pathogen: fungus (Colletotrichum). The occurrence of orchid anthracnose is first related to weather. Hot, high-humidity weather makes it most likely to occur. In early summer, with more rain and high temperatures, the disease is severe, especially in stuffy, hot environments. At first, black spots appear on the leaves, then the area gradually expands and becomes sunken, often with concentric rings. The boundary with healthy tissue is yellow or grayish green. In the later stage, the leaf tip or leaf edge dries up, and the dried part is generally grayish white. It can spread with wind, diseased seedlings, potting medium, orchid pots, and so on. It is highly contagious. Although it will not cause orchid death, it greatly reduces the ornamental value of the orchid plant. When the temperature is above 25°C, provide shade, pay attention to ventilation, and do not spray water on the leaves. Once a diseased plant is found, promptly cut off the affected parts to avoid it becoming a source of infection. The picture below shows anthracnose.

Second: stem rot. Pathogen: fungus (Fusarium oxysporum). It often occurs on pseudobulbs, old growths, and new growths. At first, a water-soaked appearance appears at the base of the leaves, then it gradually turns brown and black. Yellowing starts from the center leaf and spreads from the leaf base upward along the leaf surface. At the same time, the pseudobulb shrinks, yellows, and dries up. The pathogen can spread with diseased seedlings, potting medium, orchid pots, and so on, and it is highly contagious. Attention should be paid to controlling the moisture of the potting medium and to ventilation in the growing environment. Spray fungicides such as carbendazim or chlorothalonil once every 15-30 days. Once stem rot occurs, if measures are not taken immediately, the usual outcome is that the whole pot is lost. The infected pseudobulb (growth base) should be cut off immediately and disinfected with carbendazim or chlorothalonil. The orchid pot also needs to be disinfected, then replanted with new potting medium. The previously used potting medium should be discarded. If you are unwilling to discard it, it can be used after high-temperature steaming. The picture below shows stem rot.

Third: southern blight. Pathogen: Deuteromycota fungus, also called white silk disease or sclerotium disease. Infection begins in April to May, and the peak incidence is from June to August. The disease develops rapidly and causes the affected orchid plant to rot, wither, and die. The disease starts from the stem near the ground. At first there is a yellow to light brown exuding lesion, which later turns brown to dark brown and rots, and spreads across the soil surface around the roots and at the stem base, destroying the stem and infecting young leaves and roots. White mycelium is produced on the leaf sheaths and root mass; the affected parts become water-soaked, rot and soften, and blacken until the leaves wither. The pathogen spreads to the pseudobulb, and the sclerotia change from white to reddish brown to tea brown and necrotic. In severe cases, the entire leaf cluster dies. It is contagious and can spread through flowing water, irrigation water, rain splash, and fertilization. When the soil is acidic, the disease is most severe. Hot, humid weather easily induces the disease. Prevention methods: pay attention to ventilation and raise the pH of the potting medium. This can be solved by scattering plant ash, and fungicides such as carbendazim, chlorothalonil, thiophanate-methyl, and potassium permanganate can also be used.

Fourth: soft rot. Pathogen: bacterium (Pectobacterium carotovorum, formerly Erwinia carotovora). It mostly occurs on newly sprouted seedlings of the year. When the disease occurs, small water-soaked brown spots first appear at the base of the orchid plant, then rapidly expand, and finally deep brown rotting patches with a foul odor appear. A gentle pull with the hand and they come off. It is also prone to occur in hot, humid weather. It is mainly infected through wounds, so attention should be paid to ventilation and avoiding wound infection. Once infection is found, promptly cut off the infected pseudobulb (growth base), disinfect it, replace with new potting medium, and repot. To prevent and control orchid soft rot, avoid or reduce the occurrence of wounds, such as cuts from division, underground shoots, and pest damage. The picture below shows soft rot!

The first 3 diseases are caused by fungi, and the last 1 is caused by bacteria. For diseases caused by fungi, use antifungal drugs; for diseases caused by bacteria, use antibacterial drugs. Only by treating the cause with the right medicine and solving the problem in a targeted way can you effectively prevent and treat it and avoid detours. The principles and methods have all been taught to you. Finally, I wish everyone can easily grow orchids well and let orchids safely pass through the hot summer! Author: Mei Fei, founder of the Orchid Bar Forum, secretary of the Orchid Association of Jingshan City, Hubei Province, and an orchid industry influencer on major self-media platforms! I publish orchid knowledge and pictures every day, taking you to understand orchids in depth and teaching you to easily grow orchids well. If you have any questions about orchids, you can ask me at any time or leave a message below. If you like it, you can follow me or bookmark and forward it to help others!