Fungal Infections to Detect, Treat and Prevent

Summer is a time when fungal infections can quickly spread through your orchids. Fungi thrive in warm, humid conditions which coincidentally are the same conditions that most orchids need to survive. So, whether you are a greenhouse grower or your orchids are grown outside, you need to be vigilant about watching for the first signs of an infection. If left untreated, a fungus can kill your plants. But don’t worry, follow these instructions below to help detect, treat and prevent to bring your plants back to good health.

Let’s talk about the most common types of fungi that may impact your orchids: ​

Cercospora
Signs
Cercospora usually presents itself on the underside of a plant’s leaves as yellow spots. Over time, these yellow spots grow and will eventually take over the underside of the leaf if untreated. By the time most growers notice Cercospora, it has spread to the topside of the leaves turning from a yellow to yellow-ish green color. As the fungus progresses, the color will change to purple-ish or even black. If left untreated, the entire infected leaf will die off and drop. Cercospora most often infects Dendrobiums but can be seen in other orchid varieties as well.
​
Treatment
Using sterile clippers, remove infected leaves and dispose of them away from your plants. Spray infected plants with Captan Fungicide, Thiophanate Methyl or Mancozeb according to the manufacturer’s instruction. Be sure when spraying that the fungicide reaches into the leaf crevices to kill fungal spores.

Prevention
Cercospera thrives in a wet environment. Place your plants so that the leaves receive air circulation and adequate sunlight to dry between waterings. ​

Botrytis
Signs
Botrytis causes small brown spots on your flowers, ruining an otherwise beautiful flower. As the fungus spreads, you will notice more spots that are larger in size. 

Treatment
Use a fungicide such as Physan 20, Daconil, Thiphanate Methyl according to the product directions. For a less toxic option, you may also use Hydrogen Peroxide in a spray bottle. You should know that spraying a fungicide will help control the fungal problem, but may also spot a flower or brown the edges.

Prevention
Improve the air circulation around your plants and remove dead plant materials (expired blooms, old pseudobulbs and dead leaves). Avoid watering flowers, and water early in the morning to allow plenty of time for excess water to be absorbed or evaporated. Also, monitor night temperatures and avoid dips below 60 degrees when possible.

---

Guignardia and Phyllosticta 
(Commonly referred to as the Thai Orchid Disease)

Signs
Guignardia and Phylloscticta are known to be the same organism which is why we have included them together. They infect Cattleyas, Dendrobiums, Oncidiums and are widely recognized for plaguing Vandas. They show up as small elongated black or dark-purple spots on a plant's leaves or pseudobulbs. Over time, the spots will grow larger and will eventually lead to a necrotic infection that causes the plant to prematurely drop the infected leaves.

Treatment
Using sterile clippers, cut and remove leaves to a point below where they show infection. This type of pruning will likely be unsightly for your plant, but it will save your orchid and other surrounding orchids in the long run. Treat plants with Dithane M45, Captan or Thiphanate Methyl using the manufacturer’s instructions for dosage and frequency. 

Prevention
These fungi are highly contagious and will spread quickly through your orchids. Separate infected plants for up to two months, if possible. As in the previous fungal prevention sections, remove dead leaves and expired blooms. Spread out your plants to improve air circulation and water in the morning to allow your orchids to dry before nightfall. ​

We hope you never find evidence of these fungal infections among your orchids. However, know that if you do catch it early, you can save your plant before it is too late. 

Remember to keep a clean growing environment, remove dead plant debris, allow for proper air circulation and water plants early in the morning. These simple suggestions will help keep the fungi away. 

Happy Blooming from Better-Gro!