Black Rot of Orchids - Causes, Symptoms, Prevention, Treatment

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 made up of several species) of fungi, belonging to the genera Pythium and Phythophthora. It manifests itself especially in long periods with a warm and humid climate, since the contagious stage of zoospores requires a liquid medium for dissemination - these zoospores being swimmers.

The species known to be the most frequent pathogens that produce black rot are Pytium unicum and Phythophthora cactorum .

The life cycle of the pathogen

The fungi that produce black rot can only live in humid environments, where water is present. They have spores capable of moving, called zoospores, which populate various environments, swimming through the water. If the aquatic environment is in contact with plant leaves, the zoospores have the opportunity to infest them, penetrating the laminar surface and starting the next stage of life, by producing black rot. In this stage, the fungi develop the vegetative component of the mycelium, represented by hyphae, which quickly invade the tissues of the affected plant, producing the visible signs of infection, small, watery, translucent spots, which spread rapidly and change their color to brown and, subsequently , in black. If they are not treated, the affected plants can infect other plants and eventually die.

symptom

When the fungi affect young plants, the attack takes place at the soil line, causing the new growth to fall and, implicitly, their death. In the case of mature plants, the disease can be installed in any organ of the plant (leaves, pseudobulbs or roots), but, in general, it is signaled on new leaves or on new growths, progressing against the direction of plant growth up to the level of the roots.

As the name implies, black rot appears in the form of dark, blackish spots or lesions, which quickly increase in size and disperse in the plant body. If they are allowed to touch the package, especially in the case of monopodial species (with only one direction of growth, such as the Vanda and Phalaenopsis genera), the pathogenic fungi will kill the plants. The affected parts may turn yellow around the infected areas, and the lesions will be soft, with a watery exudate when pressed. Black rot Cattleya

avert

Growing media, containers, non-sterilized instruments and water, sprinkler irrigation, which allows water to stagnate on the plants, represent the common ways of propagation of pathogens. In this sense, it is not recommended to allow water to stagnate on the plants for long periods. The plants that are grown outside will have to be positioned in such a way that they are at a minimum height of 90 - 120 cm from the ground, since the two pathogenic fungi are common in the natural environment and the plants can be contaminated by spraying. Good air circulation in the growing areas is a preventive factor against infections , being able to facilitate the rapid wilting of plants after artificial irrigation or rain. Thus, it is recommended to install fans in the cultivation areas, to prevent the appearance of rot. At the first signs of contamination, it is recommended to separate sick plants from healthy ones. Some breeders recommend amending the growing medium with calcium-rich fertilizers in the spring, to prevent the appearance of black rot on new growth.

Mechanical treatment

In order to stop the spread of the infection, we will resort to the excision of the contaminated parts together with a part of healthy tissue, avoiding making cuts in the diseased areas and later in the healthy ones, as this aspect will contribute to the spread of the infection. The excised area will be carefully analyzed visually, to observe if there are signs of infection in the healthy tissue, highlighted in the form of black mycelium in the laminar tissue, visible to the naked eye. In sympodial species, with several directions of growth, such as species of the genus Cattleya , it may be necessary to completely excise the leaves, cutting even in the mass of the pseudobulb, to prevent the spread of the infection. If the plants are massively infected, successive excisions will be made in the rhizome, growth by growth, until healthy tissue is observed.

After the excisions are made, the plants will 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 viable healthy tissues. One of the often used empirical recipes is cinnamon powder , applied to open wounds left after surgery. Some authors recommend mixing the powder with food oil, to ensure the active principle remains in the treatment area.

Another product recommended by breeders in this regard is StopRot , obtained from mixing Bordeaux juice with lanolin. Bordeaux mixture, a fungicidal mixture of copper sulfate and calcium oxide, acts locally against fungal infection, and lanolin has the role of both an excipient and a fixing and waterproofing agent, on the one hand preventing the washing of the active principle by the water used for irrigation , but also having the role of isolation from the external environment of the living tissue, resulting from the excision. Regardless of the treatment method chosen, it is particularly 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 represented by bathing the plant in fungicides such as Truban or Terrazole , because both have etridiazole as an active substance and are effective against fungi of the Pytium and Phytophthora genera. 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.

To protect the young plants against softening, a fungicide of your choice will be used, at the recommended dilution, immediately after their extraction from the culture containers, for the purpose of planting. The treatment will be repeated every 2 weeks.

The taxa affected by black rot are varied, belonging to various genera, both botanical and hybrid. Among them we mention Aerides , Ascocenda , Brassavola , Dendrobium , Gongora , Maxillaria , Miltonia , Oncidium , Paphiopedilum , Phalaenopsis , Rhynchostylis , and Schomburgkia , but it occurs most frequently in species of the genus Cattleya and their hybrids from the genera Brassocattleya and Laeliocattleya .