Considering the moist nature of a greenhouse, it is logical that some fungus (such as molds) would thrive among your orchids. Certain fungi go even further and have evolved to thrive in your orchids. Four particular fungi afflict orchids more than others and those are Botrytis cinerea, Phyllosticta capitalensis, Pythium ultimum, and Phytophthora cactorum. Both of these fungi enjoy cool and wet conditions, so they may be more of a problem if you’re a cool grower (though orchids that naturally grow in cooler climates are oftentimes more resistant) or near cooler parts of your growing area, such as by an open window. They also like stagnant air, so a great preventative is a gently blowing fan. Spores can be spread via water, air or transfer on an non-sterile tool or via fungus gnats.
Botrytis cinerea (photo: above, right) directly affects the flowers as small, brown spots.
Phyllosticta capitalensis (photo: below, left) attacks the leaves and is very unsightly. It’s worth noting that this can look very much like mesophyll cell collapse and the ultimate causes (cold and wet conditions) are the same for both conditions.
Pythium ultimum and Phytophthora cactorum are both causes of “Black Rot”. These are only transferred through water but once absorbed into an orchid leaf it will spread as a black blotch that quickly overtakes the entire leaf.
Crown Rot is fungus related and can only be prevented (rather than treated). Certain orchids that have monopodial growth patterns such as Phalaenopsis and Paphiopedilum should be watered carefully. The leaf axis can easily hold water but in the wild this would not happen so they are susceptible to rotting if water is left in their top. Water thoroughly but around the sides of the plant and if water does get into the leaves try to blow the water out with a quick blow of air.
|Cinnamon||Use this natural treatment mostly for prevention of fungal infection (such as on a freshly cut leaf). Cinnamon has been heralded as an effective treatment for fungus in orchids.||Safe indoors and in your food.|
|Physan 20 or Captan||For more significant fungal infections, these sterilizing chemicals should be used when fungal problems have gotten out of hand.||Both Physan 20 and Captan come with warnings for humans and should be used with careful adherence to the product’s instructions. They are not terribly toxic, but use caution nonetheless.|
|Hydrogen Peroxide||Certain orchids don’t respond well to Physan on their foliage. It has been suspected that Physan actually strips the protective coating off leaves, making them even MORE susceptable to fungal and other problems. The answer to this problem is to use Physan 20 only on the roots of the plants, either soaking, if bare-root, or in watering, and to use a solution of 30% hydrogen peroxide sprayed on the plant.||Safe indoors.|
|Athlete’s Foot Spray||At our September 2010 meeting, Ellen Covey of Olympic Orchids told us that she sprays a mist of generic athletes foot spray above her orchids with very positive results. She does this on her seedlings as well as her mature plants of all types.||Safe indoors.|