Alternaria Leaf Spot: Identification, Mulch Suppression & Fungicide Rotation

Alternaria leaf spot, caused by Alternaria species (A. brassicae, A. brassicicola on brassicas; A. solani on tomato/potato; A. alternata broad host), creates target-like lesions with concentric rings. Spores overwinter on debris and seed; wind spreads them. Management combines resistant varieties, straw mulch, rotation, and FRAC-group-rotated fungicides.

Symptoms

• Small, dark circular spots (up to ½") → enlarge, develop concentric rings (target board).
• Colors: black → tan/gray with yellow halo.
• Sporulation: fuzzy texture on mature lesions.
• Leaves yellow, wilt, drop; fruit lesions = unmarketable.
• Invisible internal infection → don't eat spotted produce.

Integrated Management

1. Resistant Varieties (First Line)

• Many brassica, tomato, potato cultivars have partial resistance—choose them.

2. Straw Mulch (Proven Suppression)

• NYSAES trials: straw mulch at planting significantly reduced Alternaria severity vs. black plastic, biodegradable plastic, or bare soil.
• Straw-mulched plants also grew taller.
• Mechanism: reduces rain splash, moderates soil temp/moisture, may host antagonists.

3. Crop Rotation (4 Years Ideal)

• Alternaria species are often host-specialized. 4-year rotation with non-host families breaks cycle.
• Control host weeds in rotation years.

4. Sanitation

• Remove fallen leaves, spent plants promptly.
• Don't save seed from infected crops (seed-borne).

5. Fungicide Program (When Needed)

• Organic: Copper (M1), captan (M4)—limited efficacy, frequent application.
• Conventional: Rotate FRAC groups: chlorothalonil (M5), iprodione (2), boscalid (7), penthiopyrad (7), difenoconazole (3), azoxystrobin (11), fluxapyroxad (7).
• Start at first sign; 7–14 day intervals per label.
• Always tank-mix or alternate single-site with multi-site (M) to delay resistance.

6. Plant Health

• Proper spacing, nutrition, irrigation—stressed plants suffer more.

Alternaria is a debris- and splash-driven disease. Straw mulch, rotation, and smart fungicide rotation keep it manageable. Skip the mulch or the rotation, and the target spots will return every year.