Fungicide Resistance in Stemphylium Leaf Blight: Threats and Management Strategies

Fungicide Resistance in Stemphylium Leaf Blight: Threats and Management Strategies

Introduction
Stemphylium vesicarium, the causal agent of Stemphylium Leaf Blight (SLB), poses significant fungicide resistance risks due to its short asexual cycle, multiple generations per season, and prolific spore production (Gossen et al., 2021). This article examines resistance patterns across FRAC groups and offers management solutions for onion and pear growers.

Current Resistance Status by FRAC Group

1. FRAC 2 (Iprodione)

• Documented resistance in pear brown spot (Alberoni et al., 2005)

• Associated with histidine kinase gene mutations (Alberoni et al., 2010)

• Field efficacy declines in NY onion fields since 2018 (Hoepting, 2019)

2. FRAC 3 (DMI Fungicides)

• Reduced efficacy of difenoconazole/propiconazole in NY (2020 trials)

• 90% sensitivity in Ontario lab tests (Stricker, 2021)

• Resistance emerging in northeast US onion production

3. FRAC 7 (SDHIs)

• Variable sensitivity to fluopyram/fluxapyroxad vs. boscalid

• 50% boscalid resistance in NY isolates (Hay et al., 2019)

• Shifting fluopyram sensitivity in field populations

4. FRAC 9 (Anilinopyrimidines)

• Reduced field efficacy in NY and Ontario

• Documented cross-resistance patterns

• Common in NY onion production programs

5. FRAC 11 (QoIs)

• High azoxystrobin resistance (70% NY isolates)

• G143A cytochrome b mutation confirmed

• Complete cross-resistance with pyraclostrobin

Alternative Chemistry Options

Source: Brunelli et al., 2014; Ontario Ministry of Agriculture, 2021

Resistance Management Strategies

1. Rotate FRAC groups – Avoid consecutive applications from same mode-of-action

2. Mix active ingredients – Use multi-site fungicides like captan

3. Monitor field efficacy – Annual sensitivity testing recommended

4. Cultural controls – Improve air circulation, reduce leaf wetness periods

PhosphoGuard” Systemic Quad-Action Formula

Key Features:

1. Quadruple Action: Fungicide + Bactericide + Resistance Inducer + Foliar Fertilizer

2. Target Pathogens:

   • Fungi: Downy/Powdery mildews, Scab, Root rots (Phytophthora, Pythium)

   • Bacteria: Fire blight (Erwinia), Xanthomonas leaf spot

3. Unique Mechanisms:

   • Blocks fungal growth/sporulation

   • Boosts plant immunity via phytoalexins and cell wall reinforcement

4. Nutritional Benefits:

   • Phosphorus/Potassium-rich foliar feed

   • Enhances soil mycorrhizal activity

5. Application:

   • Dosage: 200-300ml/100L water (0.2-0.3%)

   • Frequency: 2-3 applications at 7-10 day intervals

Ideal For: Citrus, pomegranate, olive, strawberry, and field crops

For current fungicide recommendations:
Ontario Crop Protection Hub