Late-Season Pulse Crop Diseases & Management – Comprehensive Notes

Fava Bean (Vicia faba)

• Long-season pulse (typical maturity: $105\text{–}120\;\text{d}$; most varieties $105\text{–}110\;\text{d}$).
• Best adapted to moist, cool environments (Northern & Eastern Saskatchewan).
• Highly tolerant of saturated soil; partial resistance to Aphanomyces root rot.
• Biological N-fixation: up to $\approx 80\%$ of total N requirement, highest fixation rate among pulses yet demand more N per bushel than canola.
• Seed traits: highest protein, very low starch ➜ feed & fractionation (human food) markets.
• Anti-nutritional vicine/convicine ➜ causes favism (hemolytic anemia) in susceptible people; breeding now focused on low-vicine lines to expand markets.

Current Saskatchewan Foliar Disease Landscape (2022 survey)

• 7 voluntary fields sampled; DNA (qPCR) results:
– Sclerotinia: detected (≥1 sample).
– Chocolate spot (Botrytis fabae): 1 positive.
– Ascochyta fabae: 6 positives.
– Alternaria: all samples positive.
• 2023 season: unusually early flowering (≈ 15 Jun) ➜ earlier disease risk; first chocolate-spot samples arrived in June.

Key Foliar Diseases

1 Chocolate Spot (Botrytis fabae & B. cinerea)

• Most yield-limiting pathogen of faba bean; traditionally “late-season,” but can infect any stage.
• Economic damage window: early flower → mid-pod.
• Symptoms:
– Early: small red-brown “chocolate syrup” specks scattered over lamina.
– Progresses to large grey → black necrotic lesions; infects leaves, stems, pods.
– Plant stress response → tissues turn black, complicating diagnosis.
• Biology:
– Survives as sclerotia on residue/soil.
– Spring: conidiophores on sclerotia produce conidia; dispersed by rain-splash & wind.
– Infection starts low canopy, moves upward; favoured by >25^{\circ}\text{C}, high humidity, dense canopy.
• Severe flowering infections ➜ flower abortion/pod loss; seed infection leads to next-year carry-over.

2 Stemphylium Blight (Stemphylium spp.) — emerging

• Previously a lentil disease; first confirmed on faba ≈ 2–3 yr ago (species ID ongoing).
• Highest prevalence in wet years; both seedborne & residue borne.
• Lesions start on leaf margins, large grey-black zones moving inward; easily confused with chocolate spot when small.
• Later onset than other blights; mainly hurts seed quality (size, coat staining) more than yield.
• Wind-borne; rotate ≥$4\text{–}5\,\text{yr}$ between faba/lentil fields; monitor neighbouring field proximity.

3 Ascochyta Blight (Ascochyta fabae)

• Intermediate host-range: primarily faba but can affect soybean, dry bean, clover, vetch.
• Residue & seedborne; spread by wind, rain splash, plant-to-plant contact.
• Cool, wet July–Aug $\Rightarrow$ greatest risk.
• Lesions: small dark-brown spots on both leaf surfaces, sharp dark margin; with time, centres turn grey/tan with black pycnidia.
• Late infections: “shot-hole” leaves, stem breakage, lodging, shrivelled & stained seed.
• Yield loss unknown but quality loss well-documented.

Integrated Management for Faba Bean Foliar Diseases

• Priority tool = timely scouting. Start at early flower ➜ full bloom (normally early–late July; earlier in 2023).
• Fungicide: mid-flower (≈ 2–4 flowering nodes) provides best ROI; later sprays may be uneconomic.
• Late chocolate-spot infection can actually hasten maturity in northern areas (sometimes desirable).
• Always scout past flowering to assess pod/seed infection & plan seed selection.
• Rotation, residue management, certified disease-free seed, and avoiding dense, overly-lush canopy critical.

Soybean (Glycine max) – Saskatchewan Context

1 Phytophthora Root & Stem Rot (PRR)

• Pathogen: Phytophthora sojae (oomycete; soybean-specific).
• Can strike any growth stage; seed treatments & resistant cultivars provide early protection.
• Late-season symptom: chocolate-brown lesion girdling stem from soil line upward; foliage wilts/yellows; infected plants may appear singly/patchy among healthy ones.
• Favoured by heavy, compacted, saturated soils + warmth.
• Management:
– Rotate to non-host ≥$4\,\text{yr}$.
– Choose Rps-gene resistant varieties.
– Improve drainage & reduce compaction.

2 Iron Deficiency Chlorosis (IDC)

• Nutritional disorder, not pathogen; aggravated by high soil salinity & carbonates.
• Symptom: interveinal yellowing (score $1 \to 5$). Yield penalty ≈ $2.5\text{–}3\;\text{bu/ac}$ per $0.1$ IDC score increase.
• Mitigation: IDC-tolerant varieties, high seeding rates, good drainage; foliar fungicide ineffective; iron chelates variably successful.

3 Anthracnose (Colletotrichum truncatum)

• Present every year at low levels; major losses rare in SK (semi-arid).
• Signs: tiny black setae on tissue (10× lens), brown cankers on petioles, pod infection ➜ fewer/shrivelled seeds; brown veins, leaf curl.
• Symptoms often appear only at/after maturity even though infection can occur earlier.
• Overwinters on residue up to $4\,\text{yr}$; spreads via rain splash.
• Fungicide timing: early flower (preventative).

Dry Bean (Phaseolus vulgaris)

Bacterial Blights

Halo & Common Bacterial Blight (Pseudomonas syringae pv. phaseolicola & Xanthomonas axonopodis pv. phaseoli)

• Water-soaked/greasy spots $\to$ tan lesions with light-green/yellow halo.
• Pod lesions = small brown scabs.
• Halo toxin produced when T < $21^{\circ}\text{C}$ part-day.
• Seedborne & splash/wind/irrigation/hail spread; rapid epidemics under irrigation.
• Control:
– Certified disease-free seed + seed treatment.
– $\ge 4\,\text{yr}$ rotations.
– Bury debris (only when soil dry).
– Avoid overhead irrigation/over-watering.
– Copper-based products may slow (bactericidal); standard fungicides ineffective.

White Mold / Stem Rot (Sclerotinia sclerotiorum subsp. de Baryi)

• Requires dense canopy, high humidity, moderate temps.
• Symptoms: light-brown water-soaked lesions; cottony mycelium at stem base; plants may die within days.
• Management: residue burial, avoid post-closure irrigation, wider row spacing, rotation, fungicide at early bloom (preventative).

Lentil Link (quick note)

• Stemphylium blight in lentil mirrors faba descriptions: top-canopy necrosis ➜ leaflet drop ➜ pod loss; scout beyond bloom; rotate $4\text{–}5\,\text{yr}$.

Chickpea (Cicer arietinum) “Health Issue” – Unknown Etiology

Field Observations (since $\approx 2019$)

• Symptoms inconsistent, often no classic lesions. Reported forms:
– Leaf-tip/leaf-margin scorch or necrosis.
– Wilting, necrotic apical growth.
– Branch die-back (“leaf branch in-death”).
– Whole-plant death in patches vs. isolated plants.
• Appears flowering → early pod (≈ mid-July window; earlier in 2023 due to early season).
• Initially variety differences (Orion > Leader, 2019) but now seen across Orion, Leader, CDC Consul etc. (Kabuli types).

Differentiation from Fusarium Wilt/Root Rot

• Classic Fusarium oxysporum wilt = bottom-up chlorosis, lime-green stem, brown curling leaves, distinct red vascular streak in pea; for chickpea, root cortex darkens from inside-out rather than external blackening.
• 2023 samples:
– Leader area: dying plants positive for F. oxysporum; healthy plants negative.
– Moose Jaw: all plants carried F. oxysporum, F. redolens, F. solani; dying plants also had F. avenaceum (more virulent).
– Healthier zones had higher OM, better porosity; compaction worsened disease.

Ongoing Multidisciplinary Investigation (SPG, AAFC, U of S, U of M)

• 5 chickpea sites with weather stations + soil moisture probes (Crop Intelligence) – “Chickpea 3.0”.
• Weekly sticky traps: insect vector survey (Dr. Sean Prager).
• Research angles: environment, root rot complex, microbiome, nematodes, nutrition, herbicide residue, viruses.
• Still a “mystery”; 2024 focus may shift to broad on-farm mitigation “trial-and-error” to identify workable remedies.

Best Management Practices for Late-Season Pulse Diseases

• Scouting: Pre-flower, flowering, post-flower, and pre-harvest; assess stem/pod/seed for latent issues.
• Seed Quality: recognise seedborne thresholds (e.g., Ascochyta in chickpea $\le 0.3\%$); infected pods often mean unusable seed for next year.
• Product labels & PHI: consult “Keep It Clean” spray-to-swath tool; ensure PHI (e.g., $\ge 14\,\text{d}$ for many fungicides).
• Avoid late strobilurin sprays that re-green crop & delay harvest.
• Rotation: $\ge 4\text{–}5\,\text{yr}$ between same host group where possible; remember white-mold risk from pulses ➜ next-year canola.
• Residue management: bury or manage stubble to reduce sclerotia & pycnidia carry-over.
• Canopy/irrigation management: avoid excessive N or irrigation creating dense, humid canopy (white mold, chocolate spot, bacterial blights).
• Variety & seed treatment choices: leverage genetic resistance (e.g., Rps genes in soybean, low-vicine faba, etc.) and bactericidal seed treatments where relevant.
• Economic threshold: weigh cost vs. benefit; late infections may have negligible yield impact but could affect grade/marketability.

Quick Numerical / Formula Recap

• Faba maturity: $105\text{–}120\;\text{d}$.
• Faba N-fixation: $\approx 80\%$ requirement from atmospheric N.
• IDC yield penalty: $\Delta Y \approx 2.5\text{–}3\;\text{bu/ac}$ per $0.1$ IDC score.
• Rotation recommendation: $\ge 4\,\text{yr}$ for most residue-borne foliar pathogens.
• Seedborne Ascochyta maximum (chickpea): $0.3\%$ infected seed.

Take-Home Messages

• Late-season pulse pathogens thrive on warm + moisture + dense canopy – monitor forecast & crop stage.
• Early diagnosis (scout lower canopy!) distinguishes look-alikes (e.g., chocolate spot vs. Stemphylium).
• Fungicides are largely preventative; optimum timing = early–mid flower; late sprays seldom pay and may delay maturity.
• Quality effects (seed staining, downgrades, bin-run seed viability) often outweigh direct yield loss.
• The chickpea “mystery” remains unresolved – current evidence implicates complex root-rot interactions and/or other biotic/abiotic stresses; research continues.