POWDERY MILDEW in SA vineyards

At the time, he was an MSc student waiting for the next production season in order to collect data for his fungicide analysis. This meant that he had time on his hands to research the biology of the powdery mildew fungus, as well as how this pathogen overwinters in Western Cape vineyards (a phenomenon that was poorly understood at the time).

After immersing himself in scientific literature, he knew exactly what to look for: chasmothecia, the pathogen's tiny, overwintering "survival structures" that are visible as dark, round, resting spheres under a microscope.

Grapevine powdery mildew is an obligate pathogen that needs green tissue to survive. It also needs to protect itself if it is to survive the winter. The pathogen does this by employing two reproduction phases: asexual (flag shoots) and sexual (chasmothecia), depending on the mating types present.

Surprisingly, Halleen found only a few examples of immature chasmothecia (one or two per 100 leaves, across three vineyards).

"Back then, we examined thousands and thousands of vine leaves, looking for these structures," he recalls. "Eventually, we did find some [in vineyards] in a few places in Stellenbosch and the surrounding areas."

Over the last two decades, the picture has changed significantly. Chasmothecia of the grapevine powdery mildew fungus are now prevalent in most areas in the Western Cape that produce table grapes and wine.

Since 2015 large numbers of chasmothecia have been showing up on grapevine leaves, specifically during the later stages of the annual growing season. Each infected leaf can host up to several thousand of these hardened structures that protect the spores.

These large numbers mean that the powdery mildew fungus has successfully established this mode of overwintering under local conditions in only two decades. Interestingly, a similar change (during the same period) has been detected in Eastern European vineyards. Halleen says: "The exact reasons and mechanisms at work still have to be studied, but there seems to be a link with climate change."

Tiny spores, big problems

To the human eye, the powdery mildew fungus is visible as an ash-grey to white powdery growth on green plant tissue.

The disease causes different symptoms on leaves, canes and berries. First, yellow to green blotches appear on leaves. Later, asexual conidial spores become visible as a powdery growth on the leaf surface.

The disease also causes oily, grey blotches on green shoots and red-brown to black patches on canes. Affected berries become scarred and cracked, creating conditions for other pathogens to thrive.

New research project

Over the next three years, Halleen will be investigating the spread of this fungal disease in a new research project funded by the South African Table Grape Industry (SATI), Winetech and the ARC. He will be joined on the project by his ARC colleague Dr Minette Havenga, as well as Dr Lizel Mostert from Stellenbosch University's Department of Plant Pathology.

Together, they will reassess the source of the primary inoculum of grapevine powdery mildew (i.e. asexual or sexual reproduction). The team will also consider the pathogen's fungicide sensitivity, population structure and genetic diversity, as well as how often sexual reproduction occurs during the growing season. They will base their research on the vines in 12 vineyards with different management strategies for dealing with plant pathogens.

In August 2022, Halleen provided more detail on this planned investigation during a series of industry-related information days hosted by SATI and the South African Society for Enology and Viticulture. In his presentations, Halleen shared new insights into the epidemiology of grapevine powdery mildew and its implications for managing this fungal problem in grapes.

So how does it spread, exactly?

The fungus that causes this disease undergoes both an asexual and a sexual reproduction phase in its life cycle.

"The asexual phase is what we traditionally saw in South Africa," Halleen says. "This occurs when the fungus hibernates in dormant buds. During this phase, less genetic material is exchanged, and it also takes the fungus longer to build up resistance to fungicides."

In contrast, the sexual reproduction phase occurs when two individuals of opposite mating types are present in a vineyard to produce chasmothecia. This usually happens during late summer and autumn. Autumn rain washes the chasmothecia from the leaves down to the trunks, where they overwinter.

During the immature sexual reproduction phase, the chasmothecia are susceptible to fungicides. In spring, the dormant structures burst open after only 2.5 mm of rain, to release their ascospores (spores produced within the chasmothecia).

Halleen says there are still many unknown factors in the lifecycle of grapevine powdery mildew in the local context for the team to investigate. "At this stage, we are not yet sure what the primary inoculum is, when they release spores, and how they contribute to reduced fungicide sensitivity. These are some of the questions we will look into."

Without answers to these questions, Halleen says disease models cannot effectively predict disease outbreaks, which in turn hampers integrated management practices and the timely application of fungicides.

Figuring out fungicide sensitivity

The researchers will consider the biology of the disease-causing fungus and the impact of the use of fungicides on its genetics. "Does the fungus have fungicide-resistant genes, and if so, how widely are these genes distributed among the local population? In addition to fungicide sensitivity, other factors might also be at play in the increased spread of the fungus. This includes the susceptibility of a vine cultivar to powdery mildew, and particular climate areas."

Halleen says the project aims to determine the effect of sexual reproduction on fungicide sensitivity. "We suspect a shift in fungicide sensitivity is underway."

"Our aim is not to determine whether specific products have a reduced efficacy, but rather to evaluate biological factors that contribute to decreased management," he says. "We hope to show farmers how these tiny structures (chasmothecia) affect them directly, and then come up with practical suggestions for dealing with them."

The team will also consider control measures to deal with this fungal disease, including pre-harvest vineyard practices such as canopy management, good sanitation procedures and fungicide applications.

"There seems to have been a shift in how this fungus survives," Halleen says. "There is also a possible change underway in this pathogen's sensitivity towards certain fungicides."

"Due to climate change, spring conditions in the province are becoming wetter," Halleen says. "Our rainfall is also now a little bit later in the winter and we have a lot of it in spring. This means that conditions during bud break are now favourable for the release of the spores."

"We really need to study our own situation in order to improve management practices. An urgent and critical re-assessment of the primary inoculum in vineyards is required."

As the current rate of climate change and other environmental factors alter the local production environment, producers will likely have to pay more attention to diseases such as powdery mildew and their spread, Halleen concludes.

Featured image: White, powdery fungal growth visible on a grapevine berry