**5.1 Preventive weed management**

*Cucumber Economic Values and Its Cultivation and Breeding*

and *Cyperus spp* can carry cucumber mosaic virus [42].

**4. Critical period of weed control (CPWC) in cucumber**

The CPWC is a period in the crop growth cycle during which weeds must be controlled to prevent quantitative and qualitative yield losses [21]. It is the period when a crop is most sensitive to weed competition and therefore the time interval when it is necessary to maintain wed-free condition to prevent an unacceptable reduction in potential yield [23]. It denote the optimum timing of weed removal to prevent potential yield loss. From practical standpoint, crop yield losses from weed interference before or after the CPWC will be of limited interest. This means that weeds that are present before or emerge after the CPWC do not cause significant yield loss [43]. Studies conducted on cucumber [5, 44] have shown that weed infestation during the CPWC imposes irreversible loss and damage on the final yield, while weed control before or after the CPWC did not improve fruit yield compared with crops kept weed-free only during the CPWC [45, 46]. Weed control recommendations in cucumber are therefore made on the basis of the CPWC because they indicate the optimum time for implementing and maintaining weed control at reduced cost [47]. Although published research work on weed interference and CPWC in cucumber are very limited in the literature, the few available studies [4, 5, 45, 48] have shown that the CPWC in cucumber varies across environment (location, soil, climate and management), infesting plant community (species, density and population), crop (cultivar, spacing and density), growing seasons and

In the USA, the CPWC in cucumber was estimated to be between 4 and 6 weeks

after sowing [49–51]. In another study, it was determined that cucumber maintained weed-free for as little as 2 weeks after sowing (WAS) produced yield similar to the season-long weed-free treatment [52]. The author found that a single weeding either 3 or 4 WAS was sufficient to prevent yield loss for cucumber planted on 1.2 m row spacing, and concluded that no CPWC existed. Conversely, cucumber in a narrow row spacing had a 3 to 4 WAS CPWC [52]. It was reported that the CPWC for cucumber was longer at a higher plant population than in a lower plant population [52]. In Canada, the CPWC for cucumber was determined to be between 12 to 36 days after sowing (DAS) with a mixed population of common ragweed and common lambsquarters [45]. In Brazil, it was found that the CPWC was between 3 to 7 WAS [53]. Due to the disparities in the results of the CPWC from one study and location to the other, it has been recommended that critical period of weed

Weed interference in cucumber begins during the very early stages of vegetative growth [5]. Unfortunately, cucumber is not a strong competitor at the early stage, therefore weeds out grow them during the early stage of crop growth, resulting in high yield reduction [4]. Weeds that germinates at the same time as cucumber such as annual weeds like *Amaranthus spp*, *Xanthium strumarium* and *Ambrosia artemisiifolia* grows faster and maintain canopy above and below the top of cucumber. Hence, these weeds intercept photosynthetically active radiation at the expense of the crop, resulting in reduced yield [40, 41]. Furthermore, weed-inflicted shading of cucumber flowers promotes flower abortion. Although cucumber becomes less vulnerable to weed competition after the vines run out or when they become well established, the crop may take a few more weeks to close canopy. Weeds that emerge during this period may complicate harvest by concealing fruit or hampering manual picking with prickly foliage, or entangling vines, and promote fungal diseases by limiting air circulation. *Solanum carolinense* is a host for cucumber powdery mildew fungus (*Erysiphecichor acearum*), and many common weeds such as *Amaranthus spp*

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years [44–48].

Preventive measures of weed control is an important part of weed management that has gained attention among cucumber growers and weed scientist in recent time. Preventive weed control involves techniques and practices that hinders the build-up of weed species [55]. These involves clean cultivation through the use of clean water, seeds and fertilizer, and keeping the farm environment free from weeds and their seeds [56]. It is necessary to begin preventive weed control during the year before the beginning of cucumber production and use cucumber seeds free from weed seeds to promote a weed-free cucumber crop in the preceding season. The selected field must be relatively free from weed species such as nut sedges, Bermuda grass, morning glories and Johnson grass. Seed set by pigweeds, common cocklebur and other aggressive annual weeds must also be avoided as a precautionary measure to achieve a weed-free cucumber field [49]. Other preventive weed management measures in cucumber includes not growing cucumber the year after another annual vegetable with similar tillage cultivation and harvest schedules, especially in a highly infested field. Cultivation fallow can also be used to reduce the weed seed bank in cucumber fields that are heavily infested with weeds. The choice of cucumber variety can also affect the level of weed infestation. Hence, vigorous varieties with good adaptation to the prevailing local conditions and good foliage to suppress weed should be selected [49, 56]. Optimum conditions that give cucumber a competitive advantage over the weed species must be provided to prevent buildup of weed species. Fertilizer application method and timing must be manipulated in such a way that the nutrients are available to the crop rather than the weeds. In-row drip irrigation and fertigation can be used to water and apply nutrients to the cucumber and not the inter-row weeds [42]. Measures should also be taken to use water free from weed seeds. Large amount of rapidly available nitrogen, phosphorus and potassium fertilizers that can stimulate excessive weed growth in nutrient responsive weeds such as pigweeds common cocklebur, common ragweed and lambs quarters should be avoided [42]. Many weed species have higher water use efficiency than cucumber, hence flood irrigation should be avoided because they provide conducive environment for weed to flourish. Furthermore, weeds should be removed before they set seeds to avoid weed seed spread in the cucumber crop. Removing weeds in their early growth stages prevents them from setting seeds and spreading these to other areas of the field. Therefore, it is necessary to remove weeds the first time they have been noticed [42, 49, 56].
