Weed Management in Horticultural Crops

SOME PROBLEMATIC AND NOXIOUS WEEDS AND THEIR MANAGEMENT

Grasses
1. Cynodon dactylon (L.) pers. (Bermuda grass, doob)

Biology and Habitat: Cynodon dactylon is a perennial weed growing largely from rootstocks and stolons. Although it produces seeds, yet these are not important in its dispersal. Bermuda grass rootstocks from dense sod inside the soil and its stolons creep over the land, extensively. The grass grows round the year, but it is particularly vigorous under warm and moist conditions.

Control:

In the semi arid tropics, exposoure of Cynodon dactylon rhizomes to hot Sun during summer months by deep tillage of the infested land, is an effective method of controlling the weed. It takes about 7-14 days to desiccate the exposed rhizomes where after a second tillage operation to further expose the Bermuda grass rhizome is required. Bermuda grass control by summer tillage can be improved by pre conditioning it by the application of herbicide e.g. Dalapon, Diuron, Glyphosate, Amitrole-T, and Uracils. These herbicide will aid in achieving a longer lasting and more complete kill of the weed by translocating to the underground rhizomes of the weeds.

2. Sorghum halepense (L.) Pers. (Johnson grass, baru).

Biology and Habitat: Sorghum halepense is a perennial, robust grass with upto 2m high culms. It reproduces from stout, deep rhizomes and plentiful seeds. The rhizomes may be found as deep as three meter in soil, though 35-37 cm depth in more common. A piece of Johnson grass rhizome dragged to a new spot forms a new colony quickly. The food reserves in the rhizomes are lowest when the aerial shoots are 26-33 days old. At this time the Johnson grass shoots are in boot stage and about 30-35 cm in height. Ripe seeds are produced in large numbers during the winter months and are disseminated extensively with wind. Thereafter, the old grass clumps undergo dormancy. The new seedlings appear in summer and these begin forming rhizomes within 3-4 weeks. The rhizome development is fast after the plants have formed the seed heads.

Control: In humid regions effective control of Sorghum halepense has been obtained from the earlier cited tillage cum herbicide system. Cultivation should begin in hot weather when Johnson grass is 30-40 cm tall. The weed should be allowed to regrow to the same height and then treated with two sprays of Dalapon at 5-7.5kg/ha each, using 0.5% of a suitable wetting agent, at an interval of 7-10 day. After two weeks, disc the treated field. To control Johnson grass from seeds, the crops should be treated with suitable soil active herbicides.

3. Avena spp. (wild oat, jungli jai)

Biology and Habitat: There are three common species of wild oat, namely, Avena fatua L., A. ludoviciana Dur, and A. sterilis-macrocarpa L. of these, in India the first two species are of common occurrence. A. sterilis is the principal wild oat species in Africa.
Wild oat is a serious weed of mainly wheat and barley although it may infest other winter corps too. It is so because in small grains it survives by mimicry during its vegetative phase. Botanically it can be distinguished from wheat plants by its large ligules, absence of auricles and anti-clockwise twist of its leaves. (The wheat leaves show clock-wise twist). But none of these characters can be used to identify wild oat while weeding it out form the wheat (And barley) fields.

Control: Adoption of crop rotation involving non-graminaceous winter crops, where wild oat shows itself distinctly, is a practical way of reducing the weed populations. From wheat and barley plots the weed can be pulled out only after it has shown its typical panicle when large areas of weed and barley are involved, herbicides are to employ to reduce wildoat infestation. In these crops herbicides such as diallate, triallate and barban were recommended for wild oat control in the past. In recent years, superior herbicides e.g. clodinafop, fenoxaprop, mesosulfuron and tralkoxydim have replaced the older herbicides.

4. Phalaris minor Retz. (Littleseed canarygrass, gullidanda, vanri)

Biology and Habitat: Phalaris minor, an annual invasive alien, weed, has spread wildly in many wheat growing areas of the world. Like wildoat, it mimics wheat and barley plants and sets seeds at almost the same height and time as the crop. Each plant of canary grass produced about 300-460, shiny black, very small, flat seeds which readily contaminate the wheat seeds.

Control: Unlike wild oat, the seed of canary grass can be separated from wheat and barley seeds by sieving. Crop rotation involving broadleaf winter crop, and in certain area winter maize, offer farmer chance to hoe out this grass at a very early crop stage. From winter grain crops the canary grass plants can be pulled out only when they first shows their foxtail-like spikes in Feburary,peeping through their flag leaves. Certain herbicides have exhibited adequate selectivity between P. minor and wheat (or barley).Some such herbicides are Methibenzuron, Metoxuron, Isoproturon, and Terbutryn.

5. Striga spp. (Striga, witchweed,agia)

Biology and habitat: Striga is an annual, parasitic herb, propagating from seeds. A stimulus from a suitable host root exudates, continuously for 24-28 hours, is needed to trigger the germination process in Striga seeds. Upon germination, Striga produces 2-3 mm long radicles. The radicles then from haustoria on the nearby host roosts and continue extending their growth involving the whole root system of the crop plant; sucking water, nutrients and other food material from them. Each Striga plant may produce over a million seeds, about 30 days after its emergence. The Striga seeds are as dust particles.
There are 23 species of Striga on record in the world but, of these only three are considered notorious. These are: S. asiatica, S. hermonthica and S. densiflora.

Control: Techniques to reduce seed production in Striga have been recommended. Even hand pulling before Striga plants put forth their flowers can be helpful in this respect, but it is impracticable in cases of severe infestations. Directed applications of 2,4-D is a practical alternative to this for breaking future plantations of Striga .Its amine salt is applied at 0.5-0.75 kg/ha,2-3 times during the crop season to destroy flushes of Striga in its vegetative phase. Dicamba and bentazon need be tried as more potent alternatives to 2,4-D for Striga control.Preemergent granular Atrazine is also reported effective against Striga.

6. Orobanche spp.

Biology and Habitat: These are four major parasitic species of broomrape. These are; O. cernua on tobacco and sunflower, O.ramossa and O.aegyptiaca on tomato and like crops.Broomrape is an annual, parasitic herb propagating by seeds. The host root exudates induce germination of broomrape seeds within soil. The parasite seedling then infects the nearby host roots forming haustoria on them. Soon thereafter the broomrape emerges through the soil as pale shoots, devoid of chlorophyll. Broom rape is thus a total parasite. Its clones are fleshy at the base, and they develop several tubercles which grow into separate flowering shoots. Each broomrape plants may produce over half a million seeds in a short period of about eight weeks. The broomrape seeds disseminate by wind, birds and farm animals. They can remain dormant in soil for 2-12 years.

Control: Crop breeding for introducing Orabanche resistance has made only limited headway, thus far. Pepper is generally regarded as a suitable trap crop for some species of Orabanche. Trap cropping, however has a very limited scope for broomrape control in the present cropping systems. Hand –weeding at weekly intervals is effective in preventing its further, but it is too costly an operation.
Herbicidal control of Orabanche has been found possible with allyl alcohol applied at 0.1-3.2% spray, about 2-4 weeks after transplanting tobacco. Also, preplant soil treatment with Dalapon, Linuron, or metobromuron has been found to keep tobacco free from this parasite to acceptable levels. Orabanche shoots have been further found susceptible to Glyphosate and terbutol at their usual application rates, as well as to several vegetable oils.

7. Cuscuta spp. (dodder, akashbel)

Family: Convolvulaceae

Biology and Habitat: There are at least 170 species of Cuscutta, an invasive alien parasitic weed, in the world. Of these, C. campestris, C. reflexa and C.hyalina are most common. Cuscuta is a complete parasite, twining, golden yellow, wiry stems, rudimentary scaly leaves, and small clusters of minute, bell-shaped flowers. The dodder stems contain carotenoid pigment capable of conducting only limited phtosysthesis.Germination of dodder seeds does not require any stimulation form the host crop. They germinate readily in moist soil and produce wiry plumules.The plumules try to strike nearby plant roots and stubbles. Then they travel upward in search of a suitable host shoot, as yellow seedlings. A single dodder seedling can grow 2 km or more in length.

Control: Cuscuta is one of the very few weeds that can be controlled hand foot by adopting crop rotations; replacing its host crop with a non host crop for a couple of years. This is the most pragmatic approach to control this parasitic weed. Among the selective herbicides, dodder has been found susceptible to chloropham, pronamide, Glyphosate, dichlobenil, Fluchloralin, pendimethalin, imazaquin and metolachlor. Choice amongst these shall depend upon their specific selectivity to the host crop in question.Pronamide has been reported particularly useful in killing Cuscutta growing on Niger. It is applied at 2kg/ha, about 20 days after sowing of the crop.

8. Loranthus spp.

Family: Loranthaceae

Biology and Habitat: Loranthus is a stem semi-parasite of certain tropical and subtropical trees and bushes like, mango citrus, sapota, jackfruit and tea. In India alone about 60 species of Loranthus are on record. Of these L. longiflourus var .falcatus is the most damaging. Loranthus seeds are spread by birds to fresh tree branches.

Control: The control measure of Loranthus is to bore two rows of holes down the infected tree reaching the sap wood in each hole a mixture of 8g copper sulphate and 1g 2,4-D powder is pushed in. This treatment is supposed to free the tree from the Loranthus for a period upto 4 years.

9. Convolvulus arvensis L. (field bindweed, hirankhuri)

Family: Convolvulaceae

Biology and Habitat: C.arvensis is a vigrous, twinning or trailing, perennial herb. It reproduces by seeds and creeping roots which may be as deep as 6-9 m. Though it is an evergreen plant, in excessive drought conditions it under goes dormancy. The seeds have long dormancy period in semiarid region. It binds the crop plants and makes their harvesting extremely cumbersome.

Control: In fallow fields summer tillage with overlapping sweeps every time the bindweed plants are 14-18 days old, followed by competitive cropping with fodder crops, forms good cultural method of its control. The cultivation should begin at early bud stage of the weed. Its last growth may be treated at the early bud stage with 2,4-D at 0.5-1.0 kg/ha.
In winter grains 2,4-D has been used for long to control bindweed sufficiently well to permit acceptable crop harvests. It is applied twice during the crop season, once each at the tillering and stiff dough stages of the crop. Beside after crop harvest the left over bindweed plants are recommended to be treated with 0.2—0.3% spray of mixture of 2,4-D and Silvex (2:1).

10. Cirsium arvense (L.) Scop (Canadathistle, keteli)

Family: Asteraceae

Biology and habitat: Cirsium arvense is a perennial, dioceous rough herb, with somewhat spiny leaves. It grows up to 150 cm tall, in dense stands of as much as 45 shoots/sq m ground area. The weed has a worldwide distribution in the temperate regions. In India it is problematic in winter season crops in certain areas . It remains dormant during the late summer and rainy period. The thistle propagates through its extensive root system and seeds. Its roots may reach as much as 5m deep, though about 50% of the root biomass is concentrated in the top 30 cm soil. Root segments, as small as 0.3 × 1.2 cm in length are capable of producing new plants and a 19 days old seedling can form adventurous root buds.

Control: Lucerne and berseem (Egyptian clover) form very good smother crops for the control of canadathistle. Their multicuts weaken the thistle plants sufficiently to discourage their propagation. In open crops like chickpea and mustard, on the other hand, the farmers get sufficient opportunity to physically remove 2-3 flushes of the thistle. However, in the close row crops like the winter grains, Canada thistle control is difficult by such means and it is necessary here to resort to the use of selective herbicides. Some suitable herbicides for the purpose are 2, 4-D, MCPB and sulfonylureas. MCPB can alcobe used in pea to control this weed.

11. Lantana camara

Family: Verbenaceae

Biology and habitat: Lantana camara is a perennial, evergreen, invasive alien shrub that spread readily by its abundant seeds shallow crown buds. The weed is native to Mexico. In India, it was initially introduced in 1809 from Sri Lanka as a hardy, ornamental hedge, bearing multicolour flowers. But then it escaped to open pastures, river beds, wastelands, cultivable lands, and forests, where it easily displaced the indigenous vegetation ,thereby adversely affecting the grazing grounds and other land utilities ( Prakash,1995).The seeds of lantana camara regenerates quickly from the crown buds after cuttings, trampling, or burning, to form to dense , impenetrable thickets. Lantana leaves contain a toxic compound, lantad-C, which causes lesions and changes in blood plasma of the animals compelled to graze upon it.

Control: Biological control of Lantana has widely practiced in Australia and Hawaii in particular, although it has not proved successful in India, thus far.
Herbicidal control of lantana, though possible, has not met with great acceptance. The plant is susceptible to the commonly known brush killers. In India a spray of Glyphosate (0.75-1.0%) on the new growth of lantana occurring after burning or slashing its thickets has been found encouraging in extirpating the weed in Himachal Pradesh.

12. Ageratum spp. (billgoatweed, neela phulnu, pudeni, ujaru and ukhal buti)

Family: Asteraceae

Biology and habitat: Ageratum is an invasive alien weed that is native to tropical America. Its two species, namely A. conyzoides (L.) and A. houstonanium (mill.) are common in India. While A.conyzoides is an annual species invading both crop and non crop lands, the latter is a common perennial species infesting largely undisturbed areas likes the road sides, field boundaries, pasture land, orchards and water channels.
The perennial species of Ageratum (A. houstonanium) multiplies by stem and roots, besides numerous seeds, capable of remaining dormant in soil for up to three months

Control: Ageratum is susceptible to common herbicides used in maize and other kharif crops, like Atrazine and Alachlor. In non crop areas, it can be damaged severely with a non-selective herbicide like Glyphosate.

13. Parthenium hysterophorus L. (carrot grass, chandni)

Family: Asteraceae

Biology and Habitat: P. hysterophorus is an annual invasive alien plant, with wide amplitude of ecological adoptability; being both photo and thermo-insensitve.It reproduces it self freely from numerous seeds (5000-10000 per plant), beside its crown bud which put forth new shoots as and when the mother shoot is cut. Thus , the weed spreads by leaps and bounds in any new, neglected area. The weed is notorious for causing allergic dermatitis and mental depressions in human beings. the main toxin responsible for such effects of the weed is parthenin, present in a concentration as high as 0.33 % ( d.w.b). In dry summer months Parthenium tends to stunt its growth and remain in rosette form. It shoots up new growth in the rainy season and grows fast through the winter, attaining a height about 90 cm, with profuse branching. The plant flowers and sets seeds throughout its growing period.

Control: Chemical control of Parthenium infestations is found scientifically possible with certain herbicides, particularly, 2, 4-D esters (2-5kg /ha), Diquat (0.5-1.0%) Glyphosate (0.5-1.0%), chlorimuron (0.02-0.04% kg/ha), metribuzin (0.3-0.5%) and metsulfuron (0.035-0.045% kg/ha).Of these herbicide, metribuzin is the latest recommended against P. hysterophorus.
Finding that the major P. hysterophorus problem ca not be addressed to with the herbicides for economic and feasibility reasons, the scientist laid stress on its biological control. After testing different possible bioagents for several years. Mexican beetle Zygogramma bicolorata was finally accepted as the most effective bioagents for damaging the established carrotgrass infestations on the vast non crop lands of the country.

14. Solanum elaeagnifolium Cav. w. (wild brinjal, silver leaf nightshade, white weed)

Family: Solanaceae

Biology and habitat: silver leaf nightshade is a deep rooted, perennial weed with erect and branched stems covered with fine hair and orange colour prickles. Its leaves are silvery white due to numerous white hairs. The flower is purple to violet. The plant grows profusely during the rainy season, infesting roadsides, garden lands and neglected dry lands. Its root pieces can give rise to new plants In India, the weed is presently noxious in her southern states .The weed is native is South West USA and Mexico. It is poisonous to livestock.

Control: The weed has been found susceptible to 2, 4-D and Glyphosate but it regrows after about 2 weeks of the treatment. Further research has shown that dicamba can translocate to the root of this weed and, thus, it holds good promise against it (Velu and kempuchetty,1991).

15. Cyperus spp. (nut sedge, motha)

Family: cyperaceae

Biology and habitat: There are two common species of nut sedge, namely, Cyperus esculantus L. (yellow nut sedge) and C. rotundus L. (purple nut sedge). A purple nut sedge possesses a prominent basal bulb just below the ground level. This basal bulb produces a chain of tubers which ramify as deep as 60cm in soil. But in yellow nut sedge, instead of a basal bulb, there are crown buds a little below the soil surface which gives rise to cluster of short rhizomes ending to small tubers. When mother shoot of yellow nut sedge are destroyed by tillage, new Arial shoots are borne by these crown buds. In purple nut sedge, on the contrary, the new shoots arise from the tubers.
The chief mode of propagation of yellow nut sedge if fro seed, which are 90-95% viable. Its tubers are small and slow growing. In variance with this, the purple nut sedge seeds are of very low viability (2-10%) but its tubers are prominent and grow rapidly. During the first one month growth of purple nut sedge, a mother tuber can produce four daughter tubers and in three months the tuber population may reach almost 100. This makes purple nut sedge much more problematic than yellow nut sedge.

Control: Nutsedge is found susceptible to tillage. In summer fallows, hot weather cultivations can desiccate the tuber to death. Crowbar and overlapping sweeps are good for this purpose. A tuber exposed to summer Sun in the arid and semi arid tropics, is desiccated in about 14 days if the soil was completely dry. The nut sedge tuber desiccation can be greatly improved by supplementing tillage with 2,4-D (2-4 kg/ha) at the last cultivation before the onset of monsoon. Several herbicide like 2,4-D, amitrole-T, liquid Atrazine, bentszon,salt of MAA, Paraquat, Propanil and Linuron are effective on nut sedge shoot but they seldom achieve extended control of weed beyond 1-2 weeks. These herbicides are employed for post emergence treatments in specific crops. Glyphosate is much more effective in this respect since it is translocated to its tuber to checks its regeneration (Thakur et al., 1993).