Powdery mildew is a common disease that affects strawberries and is caused by a fungus called Podosphaera aphanis. It thrives on dry leaf surfaces when weather is mild (between 60° to 80° F) and humidity is high.

What does it look like?

Early in its development, powdery mildew can be easily overlooked. Infections usually start on the undersides of leaves as small patches of white, fluffy growth. As it spreads, symptoms begin to appear as curling and purple-brown blotching of the leaves (Fig. 1). In severe cases, powdery mildew can completely cover entire leaf surfaces and even fruit (Fig. 2). This leads to a reduction in the plant’s ability to photosynthesize, thus reducing both fruit yield and quality.

Figure 2. Heavy sporulation on foliage under greenhouse conditions (left) and on an in-field fruit (right) (photos by G. J. Holmes).

Greenhouses and plastic tunnels tend to have more problems with this disease than open fields. That’s because this fungus thrives in the protected, humid environments these structures provide. In addition, protected environments filter out natural sunlight (especially UV rays) which would normally inhibit the fungus somewhat.

Where does it go in the winter?

When temperatures begin to drop during winter months, the fungus doesn’t disappear but instead “overwinters” to survive. In California, the fungus mostly survives in the form of thread-like growth (called mycelium) on living leaves. But it has another trick up its—sleeve something called chasmothecia.

Chasmothecia (pronounced kaz-moe-thee-sia) are tiny, dark specks that appear among the white powdery growth on infected leaves (Fig. 3). These structures help the fungus survive under unfavorable conditions and may be more common in colder regions. Here in California, we don’t see them as often, likely due to our mild climate and the presence of suitable hosts year-round.

Figure 3. Powdery mildew colony growing on the underside of a strawberry leaf. To the naked eye, chasmothecia appear as black specks amid white mycelial growth (left). With further magnification, you can see individual chasmothecia, which appear as dark brown spherical structures (right) (photos by M. Gutierrez).

What are chasmothecia, and why should growers care?

Inside chasmothecia are the fungus’s sexual spores, called ascospores (Fig. 4). These are different from the more common asexual spores, known as conidia (Fig. 5), which are the spores that typically spread the pathogen on wind currents during the growing season.

You might be thinking, “Why does this matter to a strawberry grower?”

Here’s the key: Think of chasmothecia as the fungus’s way of “shuffling the deck.” Inside these little survival pods, the fungus mixes its genes, creating new combinations, leading to genetic diversity. That’s bad news for growers because these changes can help the fungus:

• Survive difficult weather
• Develop resistance to fungicides
• Overcome natural resistance in strawberry varieties

In other words, the more the fungus changes, the harder it becomes to control.

What can you do?

Be aware. The presence of chasmothecia is not cause for alarm, but if you notice these small black dots amid white powdery growth on the underside of leaves, you’ll know that it’s the pathogen’s way of surviving harsh conditions and adapting to its environment through greater genetic variation. As always, regular scouting, early detection, cultivar resistance and the rotation of effective fungicides are your best tools for managing powdery mildew.