A pathogen with major destructive potential in orchid crops, but also present in other families, black rot is caused by two species (according to some authors, by a complex formed of several species) of fungi belonging to the genera Pythium and Phytophthora. It manifests especially during prolonged periods of warm and humid climate, as the contagious stage of zoospores requires a liquid environment for dissemination – these zoospores being swimmers.
The species known as the most frequent pathogens causing black rot are Pythium unicum and Phytophthora cactorum.
Pathogen life cycle
Fungi that cause black rot can only live in moist environments where water is present. They have spores capable of moving, called zoospores, which populate various environments by swimming through water. If the aquatic environment is in contact with the plant leaves, the zoospores can infest them by penetrating the laminar surface and starting the next life stage by producing black rot. At this stage, the fungi develop the vegetative component of the mycelium, represented by hyphae, which quickly invade the tissues of the affected plant, producing visible signs of infection: small, watery, translucent spots that spread rapidly and change color to brown and later to black. If untreated, affected plants can infect other plants and eventually die.
Symptoms
When fungi affect young plants, the attack occurs at the soil line, causing the fall of new growths and consequently their death. In the case of mature plants, the disease can settle in any organ of the plant (leaves, pseudobulbs, or roots), but it is generally reported on new leaves or new growths, progressing opposite to the direction of the plant's growth down to the roots.
As the name implies, black rot appears as dark, blackish spots or lesions that rapidly increase in size and spread throughout the plant. If these reach the crown, especially in monopodial species (with a single growth direction, such as the genera Vanda and Phalaenopsis), the pathogenic fungi will kill the plants. The affected parts may yellow around the infected areas, and the lesions will be soft, with a watery exudate when pressed.
Prevention
Growing media, containers, unsterilized tools and water, watering by spraying, which allows water to stagnate on plants, are common pathways for pathogen propagation. In this regard, it is not recommended to allow water to stagnate on plants for extended periods. Plants grown outdoors should be positioned so that they are at a minimum height of 90 - 120 cm from the ground, as the two pathogenic fungi are commonly present in the natural environment and plants can be contaminated by splashing. Good air circulation in growing areas is a preventive factor against infections, facilitating the rapid drying of plants after artificial irrigation or rain. Therefore, it is recommended to install fans in growing areas to prevent the occurrence of rots. At the first signs of contamination, it is recommended to separate diseased plants from healthy ones. Some growers recommend amending the growing medium with calcium-rich fertilizers during spring to prevent the appearance of black rot on new growths.
Mechanical treatment
To stop the spread of infection, the contaminated parts will be excised along with a portion of healthy tissue, avoiding cuts in the diseased areas and then in the healthy ones, as this would contribute to the dissemination of the infection. The excised area will be carefully visually analyzed to observe if there are signs of infection in the healthy tissue, highlighted as black mycelium in the laminar tissue, visible to the naked eye. In sympodial species, with multiple growth directions, such as species of the genus Cattleya, it may be necessary to completely excise the leaves, cutting even into the pseudobulb mass, to prevent the spread of infection. If the plants are heavily infected, successive excisions will be made in the rhizome, growth by growth, until healthy tissue is observed.
After performing the excisions, the plants should be placed in a well-ventilated environment to allow the excised area to dry.
Chemical treatment
After removing the infected parts of the plants, it is recommended to apply a fungicide to protect the healthy viable tissues. One of the commonly used empirical recipes is cinnamon powder, applied to the open lesions left after the surgical intervention. Some authors recommend mixing the powder with cooking oil to ensure the retention of the active ingredient in the treatment area.
Another product recommended by growers in this regard is StopRot, obtained by mixing Bordeaux mixture with lanolin. Bordeaux mixture, a fungicidal blend of copper sulfate and calcium oxide, acts locally against fungal infection, while lanolin serves both as an excipient and as a fixing and waterproofing agent, preventing on one hand the washing away of the active ingredient by the water used for watering, but also isolating the living tissue resulting from the excision from the external environment. Regardless of the chosen treatment method, it is extremely important to cover the entire excised area with a fungicide or a substance with insulating properties. Another treatment used in cases where the disease is detected early is soaking the plant in fungicides such as Truban or Terrazole, as both contain etridiazole as the active substance and are effective against fungi from the genera Pytium and Phytophthora. For more advanced cases, the use of systemic antifungals such as Aliette or Subdue is recommended. Other products indicated for the treatment of black rot are Captan, Dithane M-45, Folpan, Funguran, Curzate Manox.
For the protection of young plants against softening, a fungicide of choice will be used at the recommended dilution immediately after removing them from the growing containers, for planting purposes. The treatment will be repeated at 2-week intervals.
The taxa affected by black rot are varied, belonging to different genera, both botanical and hybrid. Among these we mention Aerides, Ascocenda, Brassavola, Dendrobium, Gongora, Maxillaria, Miltonia, Oncidium, Paphiopedilum, Phalaenopsis, Rhynchostylis, and Schomburgkia, but it most frequently occurs in species of the genus Cattleya and their hybrids from the genera Brassocattleya and Laeliocattleya.
The treatments available at Secret Garden can be ordered here (link).
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