Powdery mildew: identification, monitoring and fungicide resistance management

Powdery mildews (family Erysiphaceae, 900 species) are obligate parasites of living plants that attack nearly 10,000 species of flowering plants, including cereals, grapes, fruits and vegetables [1]. This guide covers cucurbits (Podosphaera xanthii), grapevine (Erysiphe necator), strawberry (Podosphaera aphanis) and tomato (Leveillula taurica, Oidium neolycopersici) [1]. Infected cucurbit leaves die early, leaving fewer or smaller fruit of poor quality [2]; in grapevine the disease can destroy clusters and reduce yield and quality [3]. Because programmes involve many sprays per season, the risk of resistance is high [1].

1. How to recognise it

Look for white, powdery growth on leaves, stems and sometimes fruit: chains of spores (conidia) on fungal threads that grow over the surface and feed through haustoria inserted into the outer cells [1][4]. Late in the season, tiny dark spheres (chasmothecia, the sexual fruiting bodies) may form in the colonies [2][5].

On cucurbits the growth appears on both leaf surfaces, usually first on shaded lower leaves and undersides, sometimes with yellow spots above the colonies [2]. On strawberry, colonies start on the leaf undersides, leaf edges roll up and purple-red blotches appear [6]. Tomato powdery mildew caused by Leveillula taurica shows as bright yellow blotches [7], and unlike other powdery mildews it grows mainly inside the leaf [8].

Downy mildew is often confused with it but is an oomycete, not a true fungus: it forms fluffy, bluish to grey or purple growth on leaf undersides, and on grapes starts as oily spots [5]. Fungicides targeted at powdery mildew do not control downy mildew [2].

2. Symptoms and damage

On grapevine, chlorotic spots on the upper leaf surface are followed by white growth underneath that turns powdery. Berries and rachises can be covered with white powder, mature fruit show dark web-like scars, and dormant canes show red blotches [9]. Infected berries may split and later become vulnerable to bunch rots [3].

Cucurbit leaves turn dull and chlorotic, then brown and papery [10]. Leaf loss leads to sunburnt or poorly ripened fruit, low soluble solids in melon and poor storage in winter squash [2].

In strawberry, infected flowers give deformed fruit or none, and mature fruit look seedy with white colonies [6]. On field tomatoes, L. taurica kills leaves without lesions on stems or fruit, but leaf loss causes sunburn and lower soluble solids [7].

3. Disease cycle

A spore landing on a susceptible leaf penetrates an epidermal cell, forms a haustorium to take up nutrients, and the fungus then spreads over the surface and produces new spores [1]. Because E. necator lives entirely outside the plant, fungicides need not penetrate the tissue to control it [3]. Cycles are short: symptoms appear 3 to 7 days after infection in cucurbits [2], and new spores 5 to 10 days after infection in grapevine [4][9]. Free water is not needed for infection [5]; in cucurbits, infection occurs at relative humidity as low as 50%, and rain is unfavourable [2].

FRAC, the Fungicide Resistance Action Committee, rates resistance risk higher in pathogens with many short cycles and wide spore dispersal, and lists cucurbit powdery mildew as high risk [12].

4. Monitoring and when to act

In California vineyards, UC IPM advises checking the undersides of 10 to 15 basal leaves from about 20 vines 7 to 10 days after the first spring infection. Once colonies are found, the UC Davis powdery mildew risk index tracks canopy temperature: each day with at least six continuous hours between 21 and 29 °C (70 to 85 °F) adds 20 points, and at 60 points, after three such days in a row, an epidemic is under way. The index then moves between 0 and 100, falling on days without six such hours or with over 15 minutes above 35 °C. At 60 or more, the fungus reproduces about every 5 days, and spray intervals are shortened or lengthened by index [9]. Cornell names three weeks before to three weeks after bloom as the critical period for clusters [3].

For cucurbits, Cornell advises checking both surfaces of older leaves weekly from the start of fruit formation, for example five leaves at each of ten sites. In New York the action threshold is one affected leaf out of 50; starting later compromises control and favours resistance [2]. In California strawberries, treatment starts when leaf distortion and discoloration are first detected, and autumn control reduces spring disease [6].

5. Prevention

Resistance is standard in modern cucumber varieties and common in melon, but squash and pumpkin are only moderately resistant, and a decline in suppression indicates adaptation in P. xanthii. Separate successive plantings, or plant upwind of older ones [2].

In vineyards, shoot thinning, shoot positioning and leaf removal in the cluster zone improve air flow and light; avoid low, shaded, wet sites and consider less susceptible varieties [3]. Removing prunings reduces overwintering inoculum [4]. In strawberry, avoid overhead irrigation and excess nitrogen [6]. FRAC stresses that fungicides do not replace resistant varieties, good agronomy and hygiene [13].

6. Resistance is documented

FRAC rates QoI (Group 11) and MBC (Group 1) fungicides as high risk, SDHIs (Group 7) as medium to high and DMIs (Group 3) as medium; multi-site fungicides are low risk [14].

QoI resistance in cucurbit powdery mildew appeared within a few years of the first QoIs, with cross-resistance to all QoIs tested [1][15]. In grapevine it was first described in the United States and later found at high frequency in several states [1]; the G143A mutation behind it persists for years without QoI use [16]. In 2024, FRAC monitoring found high frequencies in grape powdery mildew in eight European countries [17]. QoI resistance is also confirmed in strawberry powdery mildew [15].

MBC resistance is confirmed in cucurbit and grape powdery mildews [15]. In New York, MBC and QoI fungicides are no longer recommended for cucurbits, and boscalid (SDHI) resistance has been routine since 2009 [2]; FRAC found SDHI resistance at moderate to high frequency in European cucurbit samples from 2021 to 2023 [18]. DMI resistance builds up gradually: sensitivity shifts in grape powdery mildew are reported from the United States, Chile, Europe and Australia [16]. FRAC also records resistance to Group 13 in E. necator, to cyflufenamid (U06) and flutianil (U13) in cucurbit powdery mildew, and less sensitive isolates to Group 50 [14][15][19].

7. Rotate modes of action

Count every product containing a group, alone or in mixture, and pair it with a partner from another group that controls the disease alone; two QoIs or two SDHIs together give no protection [17][18].

• Grapevine, QoI: at most 2 powdery mildew sprays per crop, preferably in mixture; solo products in strict alternation [17].
• Cucurbits, QoI: at most 3 sprays per crop and one in every three applications, never consecutive [17].
• SDHI: all are cross-resistant; in grapes at most 3 applications a year and half of all applications, no more than 2 in a row in mixture [18].
• DMI: preventively; in grapevine at most 4 applications per season, in mixture or alternation, at the full recommended rate [13].
• Group 13: in grapes at most 3 sprays a season; in cucurbits and strawberries at most half of powdery mildew sprays [19]. Group 50: at most 3 in grapes, 2 in vegetables [20].
• Multi-sites and biologicals (BM01, BM02): no FRAC limit beyond the label [21].

The table describes FRAC groups and what the cited sources say. It is not a list of registered uses. Which products may be used, on which crops, at what rate and interval is set by each country’s registration and by the product label.

8. Biological options in the same programme

FRAC treats microbial biologicals (BM02) as low risk with no limit beyond the label [21]. UC IPM advises starting Bacillus products before disease onset and dropping soft products when pressure is high, as alone they will not give adequate control [9].

Ampelomyces quisqualis strain AQ10 is a hyperparasite that attacks the hyphae, conidiophores and chasmothecia of powdery mildews; the EU peer review found sufficient efficacy in grapes, tomato, pepper and aubergine [22]. AQ10 is the basis of Mycovia™, a product in development at Huasheng, with a documented scope for powdery mildew in tomato, pepper, cucumber and strawberry. Bacillus amyloliquefaciens D747 is in FRAC group BM02 [14]; Baciva™ is based on this strain, with documented scopes including powdery mildew control in the grape and cucumber crop groups and suppression in tomato, pepper and strawberry.

9. Regional notes

China

FRAC monitoring found reduced SDHI sensitivity in cucurbit powdery mildew in a few Chinese samples, and at low frequency in Hebei and Liaoning in 2018 [18]. In Guangxi, isolates collected from 2007 to 2012 showed varying resistance to kresoxim-methyl (Group 11), most frequently on bitter gourd (63.89%), and highly resistant isolates were also resistant to carbendazim [23]. Products registered in China are listed by the Institute for the Control of Agrochemicals (ICAMA) [24].

Latin America

In Chile, powdery mildew is one of the most important grapevine diseases [25], and control relies on up to 18 sprays per season; in a high-pressure Maule Valley field, two to four sprays guided by field observation kept severity at 0.6%, against 8.5% with seven calendar sprays [26]. DMI sensitivity shifts are also reported in Chile [16]. In Brazil, Embrapa guidance limits QoIs to two applications per season against grape powdery mildew [27].

10. A practical sequence

• Before the season, list locally registered FRAC groups and any with documented local resistance.
• Use resistant or less susceptible varieties, and manage canopy and hygiene.
• Scout weekly from the susceptible stage, checking both leaf surfaces; in vineyards, follow a risk index.
• Start at the first colonies or local threshold, and protect the most susceptible stages.
• Keep site-specific groups within FRAC limits, alternate them and add a multi-site partner.
• Use biologicals early, while pressure is low.
• Check leaf undersides after each spray, never repeat a failed group, and follow the local label.

Frequently asked questions