Fungal Diseases - Anthracnose
Anthracnose (usually Colletotrichum or Gloeosporium)
Anthracnose names a group of fungal diseases, which is seen often on soft leafed Orchids. Phalaenopsis, Stanhopea, Gongora, Coelogyne, Cymbidium, Oncidium - Alliance, Zygopetalum and Masdevallia but not limited to those Genera.
On Orchids it often occurs as brownish spots, rings or blurry somewhat irregularly shaped spots. Starting (if we talk Colletotrichum) to arise from the tip of the leaf, leaving several little half-moon shaped lines and moving downwards. While new shoots are firstly not infected, they become part of the infection as the tissue starts to harden.
Maybe at the same time, but this isn’t always the case, you see blurry spots coming up on the leaves. First a few, but quickly increasing the more the plant gets infected with Gloeosporium.
The necrosis starts in the middle of an otherwise healthy looking leaf. Once the blurry spots becomes bigger, the fungus is moving on leaving a brown to black death tissue spot, surrounded by a yellow halo. Quite often the necrotic tissue than shrinks and falls out of the leaf as it decays further.
Surprisingly, Anthracnose on Orchids is caused by a fungal disease that is found commonly on Perennials more so likely in those that belong to the Family of the Daisies, as well as on all type of grasses and on most common weeds.
The so called weed mold, or Colletotrichum gloeosporoides, is likely to occur in Glasshouses, when the humidity is very high and the air becomes stagnant. One of the rater common weeds that is hosting not only this, but many other fungal diseases, is Senecio vulgaris - the Groundsel or Old Man-in-the-Spring. This weed is in the Asteraceae genera or the Family of the Daisies. Next to weed mold it also often hosts Rust and Mildew. Once manifested on the original host, fungal diseases are likely to spread elsewhere as the infection pressure becomes too strong, even for otherwise healthy plants. Fungal spores being so tiny they then freely spread by floating off in the air.
Insufficient ventilation, or if leaves are staying wet for a prolonged period, provide good growing conditions for this fungal disease; as most commonly these fungal diseases are spread in an orchid collection through a consistently high humidity, in combination with bad ventilation and air movement. Another way of spreading it is water that is sprinkling from one plant onto another - that is frequently a common form of getting the pest spreading through a collection unintentionally and with the best of intentions! An infection through the root system on the other side is also possible.
Small, or slight, damage on a leaf might be cut off and the cut can be dusted with Flowers of Sulphur or cinnamon to protect against further infections. Especially be aware to do so on Coelogyne, as some species tend to drop the cut leaf soon after the damaged parts of the leaf has been cut.
However, Anthracnose can also occur on outdoor grown orchids quite easily, as you can imagine. So if you move your collection out into the garden in spring and bring it back into the greenhouse towards the winter, it really pays off to keep the weeds in your garden under control and have your Orchids frequently protected with fungicides. You can add Seaweed (that will break down the surface tension of the water and therefore leads to better results of the fungicide) and even add fertiliser in your spray.
By doing so you leaf feed your orchids at the same time - a win - win in my eyes. Do spray your Perennial collection of Daisies, too if you do have them in your garden.
Systemic Fungicides are usually very successful in treating this disease. Although the damages on the leaf mcan’t be cured anymore, the new growth should be free of spots. It is recommended to spray pre-emptively with a systemic Fungicide.
If the growing conditions for this disease become favorable. Alternative you could spray with Copper to give your Orchid collection a protection shield. Copper is best done / applied in Spring and Autumn but be aware, Bromeliads don’t like Copper.
To decrease the incidence / appearance of new victims popping up, one must reduce the infection pressure.
Keeping the weeds under control and if lawns are nearby, keeping them short to prevent the spreading of diseases, are best practice to optimize your over-all growing conditions in your orchid collection.