Diseases of Fruit Plantation Medicinal and Aromatic Crops

Citrus gummosis

Causal organism: Phytophthora citrophthora
Symptoms

• It is a soil borne fungus. Primary colonization is on roots causing discolouration, root decay, bark degradation at collar region and leaf falling.
• Exudation of gum like substance from bark of the trunk, which cracks open, dries up and fall down in the later stages.

Etiology

• Aseptate mycelia, zoospore are the asexual spores produced in the sporangium where as oospores are sexual spores or resting/dormant spores borne in oogonium
• PSI: Dormant mycelia and oospores present in infected debris and soil.
• SSI: Zoospore spread through soil and irrigation water.

Epidemiology
Cool weather, temperature 18-22⁰C, 90-95% RH, high soil moisture, PH 6.0-7.0

There are 3 stages
1) Asexual stage: Zoospores borne in sporangium
2) Sexual stage: Oospores borne in Oogonium
3) Vegetative stage: Mycelia with haustoria

• Oospores are sexual spores and also the resting spores present in infected debris for a longer period (6-8 months). When conditions become favourable, these oospores germinate by producing germ tube, and tip of the germ tube swells to form sporangium.
• Initially sporangium is a multinucleated structure in which each nucleus starts forming zoospore wall. Once these zoospores mature, they start moving randomly, burst open the sporangium wall and become air borne.
• Air borne zoospores move to a certain distance, then they loose their flagella and form a thin circular structure which is the encystment of sporongium
• Haustoria are intercellular where as mycelia intracellular. Under advance conditions like temperature rise and low humidity, the pathogen goes for the sexual reproduction, in which male reproductive organ is the Antheridium and female reproduction is oogonium between two gametangial processes.
• Oogonium is circular in nature, eunucleated, sometimes 1 or 4 celled. Antheredium , is tubular and multinucleated. Once the union of two gametangia takes place, it is followed by Plasmogamy, karyogamy, mitosis and meiosis.

Management

• Good drainage creates adverse conditions which results in reduction in asexual reproduction and the inoculums also decreases.
• Uproot severely infected plants and replace with tolerant varieties.
• Application of Trichoderma viridae (100 g per plant.) in the soil as bio-control agent .
• Soil drenching with Bordeaux mixture (1%) and aerial spray with Copper oxy chloride (3 gm per lit)
• Use of resistant root stocks for grafting.
• Avoid low lying areas for citrus plantation.
• Avoid excess loss of N application and apply only recommended dose of potassium