Fungal Diseases - Phytophtora infestans

Fungal Diseases - Phytophtora infestans

Phytophtora infestans (aka Black rot) is mainly seen around the late Autumn and Winter seasons; however it can occasionally occur at other times of the year. Its seasonal invasion timing is not too surprising really, as the recently grown tissues will not have fully hardened off by then and the plant’s metabolism is still adapting to the lower light and cooling temperatures too.

Identifying Phytophtora

The infection begins as blackish spots that grow rapidly, while in the background the infection spreads unseen quickly through the plants entire system. The tissue becomes extremely soft and often plants start to simply fall apart, dropping leaves, and as the disease progresses the Pseudobulbs become quickly discolored and so soft they just knock over.

Phytophtora can infest almost all parts of a plant, however on Cattleya it is often seen first on older leaves that all of a sudden start to show a tell-tale dark splotch in the middle or on the base of an otherwise healthy looking leaf, but also possibly on young shoots that have just started stretching. If the infection starts on older parts, it is more than likely that the infection actually started from the base of the plant. Often the old pseudobulb starts to become soft, and because you may not have noticed this happening, it seems as though “all the sudden” the pseudobulbs discolour very quickly.

At this stage, the infection than will be already rampaging deeper inside your orchid. Young leads or shoots (bearing in mind that in nature there’s always some exception possible) usually start the infection from the tip and melt the way down. On an otherwise strong plant with hardened off tissue, it might stop there and the young lead will just dry off. Often you can pull the blackening (and increasingly becoming mushy) lead out of its sarong at the base of the plant. But still, the plant will be set back and although the infection might not have killed your orchid this time, it’s on the lurk! I noticed that previously infected Cattleya can then struggle with the next growth and those after, especially if the fungal infection stays untreated.

Causes can be many and varied, but can include:

 Consistently very high humidity

 Keeping potting mixes too wet due to decaying potting mix/bark, oversized pots and water remaining locked in as drainage is blocked or not existing

 A mix of stagnant air and low temperatures

 Spraying/misting and irrigating is one of the main ways the infestation usually spreads from plant to plant (as it is a water and air borne fungus).

Most Phytophtora species prefer high temperatures, combined with high humidity - while many other fungal diseases thrive in a cool and humid condition however, Phytophtora infestans is more likely to occur in the damp, dappled, cooler and sticky time of the year.

In greenhouses Phytophtora will thrive at the moment when it is still too warm to heat the greenhouse through fully and therefore the humidity jumps up. That’s the moment for the fungi to attack, as the plants are also going through changes in their metabolism to adjust to the minimal (decreasing) light and over all changing growing conditions. It is a disease that almost exclusively pops up on older plants.

How to prevent an outbreak!

 Plants showing symptoms must be isolated or at least removed immediately

 Stop spraying, misting or even irrigating for the moment, till you know how much of the collection is affected and can possibly be saved with a repeatedly applied (preferably systemic) fungicide. Remember that the disease will spread through water.

 Recycled and even more, if then left untreated water, will push the infestation and it will spread through your collection quickly

 Clean out your growing area and the surrounding areas, apply a systemic fungicide, and disinfect all surfaces that had been surrounding the infected plant; you’ll need to act quickly to prevent disease spread!

 Optimise the overall growing conditions, more venting, maybe start earlier heating (and venting). Add a fan or another if needed to move the air around.

 End of summer season start to apply protective fungal treatments (e.g.FreeFlo Copper or Fostonic). Also at the end of the growing season, lower the nitrogen content but try not to wait too late.

 To strengthen your orchids and to harden the tissue, add seaweed (New in stock on the website) to your watering can at each water during the growing season, much less in winter or off-season as mealybugs may be triggered again.

 Overall, try to avoid sharply peaking conditions in all directions (too hot, too cold, too wet too dry, or an unlucky mix like too cold and too wet and vice-versa), and aim to keep growing conditions more equal.