Phytophthora foot rot and root rot of citrus Photo: Wikimedia contributor · CC0 · source ↗

Phytophthora foot rot and root rot of citrus

Foot rot and feeder root rot of citrus are caused by soilborne oomycetes of the genus Phytophthora, principally P. nicotianae (= P. parasitica) and P. palmivora in Indian conditions, with P. citrophthora reported from cooler tracts. The complex is the same pathogen group that produces trunk gummosis (Citrus Gummosis) — foot rot is the basal-trunk and crown-root expression; feeder root rot is the silent below-ground decline that often precedes visible canker. ICAR-CCRI Nagpur lists this disease complex as the principal long-term threat to Nagpur mandarin and Mosambi orchards.

Identification and symptoms

Foot rot lesions appear on the bark at the crown root and lower trunk, usually within 30 cm of the soil surface. The bark becomes water-soaked, turns dark, and may exude amber gum that gives the trunk a characteristic varnished appearance. Lesions girdle the trunk over one or two seasons. Feeder root rot is less obvious: white feeder roots turn brown, soft and slough off their cortex, leaving a stringy bare stele. Above-ground symptoms are non-specific — slow growth, leaf yellowing, sparse canopy and dieback of the terminal twigs. Fruit may also rot at the stylar end ("brown rot") during heavy rain when zoospores splash onto low-hanging fruit.

Hosts and life cycle

All commercial citrus is susceptible to some degree; rough lemon (Rough Lemon Jamberi Rootstock), Rangpur lime and trifoliate orange stocks vary in susceptibility (rough lemon high, trifoliate orange low). The pathogen persists in soil as oospores and chlamydospores; sporangia release motile zoospores in free water. Infection peaks during the monsoon and post-monsoon period when the soil is saturated. Deep planting (with bud union below the soil line), mechanical injuries from interculture, and poor drainage are predisposing.

Damage and economic impact

CCRI Nagpur estimates that Phytophthora diseases of citrus reduce yields by 10-30% in affected orchards and cumulatively kill several lakh trees per year across the Indian citrus belt. In the YSR Kadapa belt (YSR Kadapa Citrus Belt), waterlogged patches and irrigation furrows that pool water against the trunk are the chronic disease hotspots.

Management

ICAR-CCRI Nagpur and the NHB advisory recommend:

Cultural: keep the bud union 15-20 cm above the soil; raise tree basins into mounds in heavy soils; provide field drainage; avoid flooding the basin and never let irrigation water touch the trunk; use drip in preference to flood.
Rootstock selection: prefer trifoliate orange, Troyer/Carrizo citrange or sour orange on heavy and irrigated soils; rough lemon is acceptable only on light, well-drained land.
Biological: pre-monsoon soil application of Trichoderma harzianum / T. viride at about 1 kg per tree mixed with farmyard manure.
Chemical: scrape the diseased bark and paint with Bordeaux paste (10:10:100) or mefenoxam paste; soil drench with metalaxyl-MZ at about 2.5 g/L (1.5-2 L per tree) at the onset of monsoon; potassium phosphonate trunk injection or foliar spray at 0.3-0.5% in chronic orchards.
Sanitation: remove and burn fallen brown-rot fruit and infected bark; tools used on diseased trees must be disinfected.

Sources

Foot Rot or Gummosis of Citrus. National Horticulture Board.
Phytophthora diseases of citrus. ICAR-CCRI Nagpur.