Etiology and control of the following Paddy diseases: Blast, Bacterial leaf blight
Blast disease of Rice:-
Pathogen:- Pyricularia grisea (P. oryzae)
Occurance:-
> Earliest known plant disease
> Also known as rotten neck or rice fever.
> Reported from 80 rice-growing countries. First recorded in India during 1918.
> Expected grain loss: 70 to 80%.
Symptom of damage:-
> Disease can infect paddy at all growth stages and all aerial parts of plant (Leaf, neck and node).
> Among the three leaves and neck infections are more severe.
> Small specks originate on leaves - subsequently enlarge into spindle shaped spots(0.5 to 1.5cm length, 0.3 to 0.5cm width) with ashy center.
> Several spots coalesce -> big irregular patches
> Blast types:-
i. Leaf Blast:-
- Severe cases of infection - entire crop give a blasted or burnt appearance- hence the name "BLAST"
- Severe cases - lodging of crop (after ear emergence)
ii. Neck Blast:- Neck region of panicle develops a black color and shrivels completely / partially grain set inhibited, panicle breaks at the neck and hangs
iii. Nodal Blast:- Nodes become black and break up.
Management:-
i. Cultural method:-
> Remove collateral weed hosts from bunds and channels.
> Use only disease free seedlings.
> Avoid excess nitrogen.
> Apply N in three split doses (50% basal, 25% in tillering phase and 25% N in panicle initiation stage).
> Use resistant variety CO 47.
ii. Chemical Method:-
> Spray any one after observing initial infection of the disease:
i. Carbendazim 50WP @ 500g/ha.
ii. Tricyclozole 75 WP @ 500g/ha.
iii. Metominostrobin 20 SC @ 500ml/ha.
iv Azoxystrobin 25 SC @ 500 ml/ha.
> Nursery stage:- Light infection - Spray Carbendazim
> Pre-Tillering to Mid-Tillering:- Light at 2 to 5 % disease severities - Apply Carbendazim @ 0.1 %. Delay top dressing of N fertilizers when infection is seen.
> Panicle initiation to booting:- At 2 to 5% leaf area damage spray Carbendazim or Tricyclazole @ 0.1 %.
> Flowering and after:- At 5 % leaf area damage or 1 to 2 % neck infection spray Carbendazim or Tricyclazole @ 1 g /lit of water.
Bacterial Leaf Blight:-
Pathogen:- Xanthomonas oryzae pv. oryzae
Symptom of Damage:-
> Seedling wilt or kresek.
> Water-soaked to yellowish stripes on leaf blades or starting at leaf tips then later increase in lengthand width with a wavy margin.
> Appearance of bacterial ooze that looks like a milky or opaque dewdrop on young lesions early in the morning.
> Lessions turn yellow to white as the disease advances.
> If the cuts end of leaf is kept in water it becomes turbid because of bacterial ooze.
Management:-
Biological Method:-
> Spray fresh cowdung extract 20% twice (starting from initial appearance of the disease and another at fortnightly interval).
> Neem oil 60 EC 3% (or) NSKE 5% is recommended for the control of sheath rot, sheath blight, grain discolouration and bacterial blight.