Diagnosing Leatherleaf Fern Decline: Nutritional Deficiency vs. Fungicide Damage

A commercial leatherleaf fern grower in Tenerife (Canary Islands, Spain) described a three-year problem: mature fronds develop edge discoloration that progresses to brownish-yellow, then reddish-brown in older fronds. Lab tests for fungi were negative. The grower used Ridomil Gold MZ (mancozeb + metalaxyl) as a foliar spray.

Initial Assessment

The symptoms don't match classic anthracnose (Colletotrichum), the most common leatherleaf disease. Fungal, bacterial, and viral causes are unlikely. Microscopic pests or nutritional disorders (magnesium, manganese) that mobilize nutrients from older to younger fronds are possible. Parasitic nematodes (Pratylenchus spp.) could also cause such decline.

Leaf Analysis Results (Follow-Up)

Sample 2 showed substantially low levels of nearly all nutrients; Sample 1 was borderline low. Causes could include fertilizer rates, soil type, chemical sprays, or root health.

Expert Colleague Input (Dr. Robert Stamps)

Dr. Stamps, a leatherleaf authority, noted:

• Metalaxyl is known to damage some ferns; the damage pattern resembles spray injury.
• Florida growers avoid metalaxyl because water molds aren't a problem on leatherleaf there.
• Mancozeb may cause nutrient imbalances.
• Full records of fungicide/fertilizer timing and rates are needed, plus tissue and soil tests.

Recommendation

Discontinue metalaxyl. Consult Dr. Stamps' production manual: Leatherleaf Fern Production Guide. A complete site analysis is essential for a definitive solution.