Tobacco

Weed Management

Orobanche Weed Management

Orobanche (Pokayilai kalan)

Introduction

Orobanche is popularly known as broomrape in English, 'Tokra' in north India, 'vakumba' in Gujarat, 'bambaku' in Maharashtra, 'pokayilaikalan' in Tamil Nadu, 'bodu' or 'malle' in Andhra Pradesh.
It is a flowering parasite on tobacco roots and occurs in all the tobacco tracts in India. It is a holoparasite and draws is nourishment from tobacco by means of haustoria attached to the roots of tobacco.
The loss in the yield and quality of leaf is very much variable from 30-70% in Tamil Nadu and 10-50% in other states.
Experiments conducted at CTRI, Rajahmundry, indicated that loss in yield varied from 20-50% depending on the time of infection and availability of soil moisture.
In the early stages of infection, symptoms of wilting, drooping and ribbing of leaves are observed in the early hours.
These symptoms are the first indication of underground infection of tobacco roots by the parasite.
Five to six weeks after planting, young orobanche sprouts emerge from the soil at the base of tobacco plants. Presence of numerous orobanche shoots around the plants in field is the most important visible symptom.
Plants attacked early in the season are generally stunted showing typical wilting of leaves. Plants attacked late in the season do not show visible symptoms of infection but the yield and quality of leaves are reduced.
Orobanche emerge in clusters, they are 15-45 cm tall, pale brown or purple in colour. Often 10-15 shoots are found attached to the roots of a single host plant.
Of the two species reported in India, viz. Orobanche cernua and O. indica, the former is more serious parasite on tobacco and it is restricted to solanaceous plants.
Some more hosts are other families, e.g. Cannabinaceae, Compositae, Euphorbiaccae and Tiliaceae. Further Capsicum annuum (chillies) and Tridax procumbens stimulated germination of orobanche seed without getting parasitised.
The seed is though reported to be viable for 2 years remained dormant in the soil for atleast 13 years.

Etiology

The causal organism, Orobanche cernua Loefl. Var. desertorum Beck. is an annual, fleshy flowering plant, erect, 15-45 cm high.
The stem is more or less round, pale-brown, solitary (or in bunches) thickened at the base, covered with scaly leaves ending in spike, calyx with 2 narrow lobes.
Corolla is a white tube with bluish or violet lobes stamens 4 epipetalous with lobed anthers, ovary superior, one-celled with numerous ovules, stigma rather big and 4-lobed, fruit is 2-valved, capsule containing numerous seeds.
Seeds are very minute, very light, approximately 1,90,000 seeds weigh 1 gram, ovoid and reticulate in black soil. Under normal growing conditions Orobanche completes its life cycle in 12-14 weeks.
Orobanche seed germinates during the second week after planting tobacco and infects the root during third week.
Till the end of fifth week, underground infection of Orobanche develops into various sizes. On the sixth week, Orobanche shoots emerge above ground.
Flowering is completed by seventh week, seed formation by eighth week and capsule drying by ninth week end.
Drying off commences by tenth week, withering of stem and dehiscence of capsules by eleventh and twelfth weeks. Studies on the germination of Orobanche seed all round the year indicated that November to February was favourable for maximum germination when the minimum soil temperature varied from 19 to 22° C.
Germination was minimum during April to June when the soil temperature was 25 to 31.5° C. O. cernua is a holoparasite and the seeds germinate in presence of roots of tobacco and other host plants to the extent of 0 to 4.6%.
Host root exudates stimulate germination 21 days at 21° C in the presence of vitamin Niacin (nicotinamide) and vitamin B-6 (Pyridoxine hydrochloride) to the extent of 45.5% and 90% respectively.
Orobanche seeds being very minute in size are produced in enormous numbers in each capsule. When the capsule is dry, it splits open at the top and the small dust-like seeds are easily scattered by wind.
They mix up with the soil and remain viable for long periods. Irrigation and drainage water carry the seeds from one field to another. It is also spread through the animals, man or cultivation equipment.
Seeds present in the soil germinate in presence of susceptible hosts like tobacco, tomato, brinjal and safflower.

Control

Prevention of formation of the inoculum potential form the basis of orobanche control. Regular weekly hand pulling of tender orobanche shoots before they set seed, has reduced the original stand by 85% after 2 years and by 96% after 4 years.
An alternative and equally effective method is spray drenching the emerged Orobanche shoots at tender stage (7-10 cm height) with Allyl alcohol at 0.1% (1g/lit) which scorches the Orobanche shoots and they wilt and die within 48 to 72 hours after application, whereas the host plants remain unaffected.
However, for orobanche shoots in advanced stage or those that appear at later stage, concentration of Allyl alcohol has to be increased upto 0.2% (2g/lit) beyond which it becomes phytotoxic to the host.
Direct application of kerosine oil 3-4 drops per shoot has given better knock down effect. Since kerosene oil is highly toxic to host, attempts are being made to develop a safe, easy and economic device for application.
Combination of both the methods, viz. removal by manual labour upto first priming of leaves followed by chemical spray is suggested. This would be more economical depending upon the intensity of infection.
Soil application of Ethylene dibromide (EDB) @ 2-5ml/m in heavy clay or light medium soil significantly reduced Orobanche infestation.
Incidence of orobanche was much reduced in tobacco following jowar or gingelly or green gram in kharif but chillies though reported to be a trap crop failed to give the same effect.
Regarding dates of planting on the incidence of orobanche it is found that late planting reduced the incidence of orobanche.

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Weed Management

Weeds are generally kept under check by summer cultivation and three or four intercultures given during period of crop growth.
When weeds become a problem they can be brought under control by use of herbicides.
In case of tobacco in general and cigarette tobacco in particular use of chemicals which leave residue on the leaves cannot be advocated.
Herbicides like 2, 4-D, 2, 4, 5- T, tafazine have some adverse effect on the chemical and physical attributes of tobacco.

Weed flora of the tobacco field

Common Name	Scientific Name
Grasses	Dactyloctenium aegyptium Beauv.
Crowfoot grass	Cynoden dactylon (L.) Pers.
Bermuda grass	Chloris barbata SW.
Mail Kondaipul	Panicum repens (L.)
Injipullu	Echinochloa colonum (L.) Link
Jungle grass
Sedges Nut grass	Cyperus rotundus (L.)
Broad leaved weeds	Orobanche sp.
Broom rape	Trianthema portulacastrum (L.)
Carpet weed	Digera arvensis (Forsk.)
Thoyya kerai	Commelina benghalensis
Kanan Keerai	Phyllanthus maderaspatensis (L.)
Melanelli	Parthenium hysterophorus
Congress grass	Amaranthus viridis (L.)
Kuppai keerai	Solanum nigrum
Keeri

Some of the herbicides recommended for tobacco

Herbicide injury

1. Post emergence (Nursery)
Fluazifop (150 g/ha)	When needed it can be combined with one hand weeding.
2. Bidi tobacco (Main field)
Oxyfluorfen (0.5 lit/ha), Fluchloralin (1.0 lit/ha)	Apply about 8 days before transplanting combine with one inter cultivation.

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Tamilnadu

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