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What Do You Need to Know About Indoor Plant Diseases-Part III

We’ve talked about some of the indoor plant diseases in the last blogs, let’s learn about more of them in this blog.

Powdery Mildew (Erysiphales)

Powdery mildew is another cosmopolitan, common plant disease, and probably the most widely recognized one. As the name indicates, you can easily recognize this disease by its typical symptom – powdery, white mycelial growth on foliage. It all begins with small white or yellowish spots on the upper side of the leaves. In favorable conditions, the fungus spreads rapidly and occupies the entire leaf surface. Even though it first occurs on older foliage, powdery mildew can also successfully grow on buds, flowers, and young shoots. Spreading mycelium causes the photosynthetic surface to shrink, and the fungus also impairs the plant’s growth by stealing its nutrients.

Powdery mildew often occurs when the air humidity level is high, and the air circulation around the plants very poor. These fungi need moisture to initiate the infection, but once they make contact with the plant, they can thrive even in dry conditions. Although it rarely kills plants, powdery mildew can decrease the quality and quantity of yields. It can also progressively deteriorate the health of perennial species with repeated infections. As it gives plants a weakened, unsightly look, the presence of this disease is especially unfavorable in ornamental gardens and landscaping.


  • Pale yellowish spots and blotches on leaves
  • Powdery spots on the upper or both sides of the leaf
  • Premature fruit ripening
  • Leaves dry and start to curl towards the upper side

As the disease progresses, the whole surface of the leaf can be covered with powdery white mycelia. The affected foliage eventually turns from yellow to brown and dries completely. This decreases plant vigor and drains it of resources that would have otherwise gone to fruit or bud production.

Gray Mold (Botrytis cinerea)

Gray mold is among the most common diseases in plant production, affecting more than 200 plant species. It is caused by a very persistent and widespread fungus called Botrytis cinerea. The name of this fungus directly translates to “grapes like ashes” which describes the shape of its reproductive organs and the gray color of the spore masses. Besides directly affecting growing plants, this pathogen can remain latent until after the harvest. Seemingly healthy stored fruits infected with B. cinerea will develop brown blotches that soon become covered in the grayish mycelium. The tight packing of the fruits enables the fungus to easily spread from infected fruits to healthy ones. This can result in severe losses of stored goods, so it is not a surprise that gray mold is considered one of the most important diseases in agriculture.

Gray mold usually develops on older growth first, causing little to no damage to the plant. However, if the conditions become favorable for its development (high air humidity, moisture, temperature between 65-75℉) the fungus will grow rapidly. It can spread onto younger growth and ripe fruits, deteriorating the plant’s health and causing yield loss. Botrytis cinerea is perhaps most famous for its detrimental effect on wine grape production, but it is also “popular” in fruit, vegetable, and cannabis production.


  • Elliptical, watery lesions on stems
  • V-shaped or circular yellow to brown spots on leaves
  • Dry, curled leaves
  • Tan or brown soft blotches on fruits that quickly become covered in gray mycelium

Gray mold spores are everywhere, and they move around easily with the help of air currents, water and insects. High levels of moisture, warm temperatures, and poor air circulation stimulate their germination and mycelial growth. The first symptoms usually appear on older growth in the form of large brown lesions on leaves and stems. If there was a lot of moisture during budding and fruit production, grey mold can develop in the crevices of buds and fruits, and decimate the yields early. On fleshy fruits, it forms dark blotches of softened tissue that is soon covered by a layer of grayish mycelium. In humid conditions, ashy mycelial growth can cover the fruits and buds entirely.

Leaf Spot (various fungi and bacteria)

Leaf spot is quite a wide term, and it encompasses many different plant diseases that share a common symptom. As the name indicates, the typical symptom includes spotting on leaves. The spots are small, initially yellow, but soon they turn yellowish-brown and have clear edges with a yellow halo. It is usually very hard to know which species of fungi or bacteria caused the disease, but you can at least differentiate between the two. If the spots are watery, the disease is probably a result of a bacterial infection. In both cases, the spots will increase their numbers as the disease progresses, grow larger and turn necrotic.

Leaf spot is mostly caused by fungal and bacterial pathogens that enter the plant through stomata and hydathodes on its foliage. This is why it manifests itself in such a way – each spot is a place where the pathogen has entered the plant and established itself. This is also the reason why it appears on older leaves first. Older foliage has stomata big enough for the pathogen to get in.

Causative agents of leaf spot only feed locally, and rarely affect other plant organs. Even though leaf spot is usually not lethal, it can reduce the photosynthetic surface of the plant and spoil the looks of ornamental species. By reducing its vigor and photosynthetic activity, it also makes the plant more susceptible to other diseases.

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