Grapefruit Brown Rot and Dieback: Fungal Disease Management

A grapefruit tree suddenly produces fruit with large, leathery brown areas and increased fruit drop. Follow-up reveals scattered leaf spots and one dead branch with greenish-yellow spots on wood.

Primary Diagnosis: Brown Rot

The leathery brown fruit lesions strongly suggest brown rot (Phytophthora spp.), a water mold (oomycete) that thrives in wet conditions. Key features:

• Firm, leathery tan-to-brown lesions on fruit
• Often starts at stylar end or where fruit contacts soil/water
• White, velvety sporulation under high humidity
• Associated with splash dispersal from soil

Secondary Concerns: Dieback and Leaf Spots

The dead branch and leaf spots indicate possible:

• Phytophthora root/crown rot: Causing branch dieback via vascular damage
• Citrus canker: Raised, corky lesions on fruit, leaves, stems
• Melanose (Diaporthe citri): Small, raised, dark spots on fruit/leaves
• Greasy spot (Mycosphaerella citri): Yellow-brown blister-like spots on leaves

The greenish-yellow spots on wood are likely lichens—harmless epiphytes.

Immediate Actions

1. Sanitation: Remove and destroy all affected fruit (on tree and ground)
2. Prune: Remove dead branch, cutting into healthy wood
3. Improve drainage: Ensure water moves away from root zone
4. Adjust irrigation: Avoid wetting canopy; water mid-morning
5. Skirt pruning: Raise canopy 24"+ above ground

Diagnostic Testing

Given multiple symptoms (fruit rot + leaf spots + dieback), laboratory diagnosis is strongly recommended. Contact your state's plant diagnostic lab via the National Plant Diagnostic Network or your county cooperative extension office. They can identify the specific pathogen(s) and recommend targeted treatments.

Next Season Prevention

• Preventive copper hydroxide sprays at petal fall and summer flush
• Phosphonate fungicides (e.g., fosetyl-Al) for Phytophthora suppression
• Maintain optimal nutrition and irrigation

Don't delay testing—accurate identification saves time, money, and the tree.