**1. Introduction**

Cucumber (*Cucumis sativus* L.) is considered the fourth most essential vegetable worldwide, perhaps due to its great nutritional, medicinal and economic potential [1, 2]. In terms of world total production, cucumber is rated an important cucurbit alongside watermelon (*Citrullus lanatus* L.) and melon (*Cucumis melon* L.) [3]. Low yield and insufficient use of the product previously contributed to ranking the crop insignificant in Africa [4]. At present, the demand for cucumber is on the increase daily in Africa because of the continued campaigns about the numerous benefits of the crop [5].

Despite the increasing relevance of cucumber, production is seriously constrained by many factors which include scarcity of suitable planting materials, limited access to capital, climatic conditions, plant pests and diseases, among others [5]. More than 40 diseases caused by viral, bacterial, fungal and nematode pathogens severely affect the cultivation and production of *C. sativus* L. [6]. Insect pests are reported to consume crops sufficient to feed an additional one billion people on a worldwide basis, thereby placing much importance on identifying and managing them prior to infestation [7].

Like other cucurbits, the most common pests of *C. sativus* L. are spotted cucumber beetle (*Diabrotica undecimpunctata*) and striped cucumber beetle (*Acalymma vitatum*) [8]. Other pests identified on cucumber fields in Africa include Flea beetle (*Phyllotreta cruciferae*), Hadda beetle (*Epilachna vigintioctopunctata*), banded cucumber beetle (*Diabrotica balteata*), squash bug (*Anasa tristis*) and squash vine borer (*Melittia cucurbitae*). Additionally, *C. sativus* L. harbours a number of aphids which include melon aphid (*Aphis gossypii*), cowpea aphid (*Aphis craccivora*), potato aphid (*Macrosiphum euphorbiae*) and green peach aphid (*Myzus persicae*), which serve as common vectors of important viral diseases of *C. sativus* L. [9].

In Africa, cucumber is largely affected by viruses belonging to three genera; namely *Potyvirus*, *Cucumovirus* and *Crinivirus* [10]. *Zucchini yellow mosaic virus* (ZYMV), *Watermelon mosaic virus* (WMV), *Moroccan watermelon mosaic virus* (MWMV), *Papaya ringspot virus* (PRSV), *Cucumber mosaic virus* (CMV) and *Beet pseudo-yellows virus are some of the viruses* that cause severe symptoms to cucumber [11]. Other viruses of minor importance include *cucurbit aphid-borne yellows virus*, *cucurbit yellow stunting disorder virus*, *melon necrotic spot virus*, *squash mosaic virus* and *tomato spotted wilt virus* [12]. Plant virus infections result to reduction in plant growth, lower yields, compromised fruit quality, reproductive difficulty, increased susceptibility of the host plant to other pathogens and economic losses to farmers [13].

Several fungi attack seedlings of cucumber, causing appreciable losses, especially under favourable environmental conditions [14]. In Africa, many pathogenic fungi such as *Alternaria tenuis*, *Alternaria alternata*, *Fusarium equisett*, *Fusarium solani*, *Aspergillus* spp., *Phytophthora capsici*, *Penicillium oxalicum*, *Bipolaris* spp., *Botrytis cinerea*, *Cladosporium tenuissimum*, among others, have been associated with rotting of cucumber fruits [15], causing reasonably high post-harvest losses [16, 17]. Downy mildew, powdery mildew and anthracnose also cause substantial losses. Scab affects aboveground plant parts, including the fruits.

Root-knot nematodes are prevalent destructive pathogens of *C. sativus* L. because even at low levels, high yield losses result [18]. Other nematodes include species of *Rotylenchus*, *Benololaimus*, *Pratylenchus*, *Paratylenchus* and *Trichodorus*. Angular leaf spot is the most common bacterial disease of *C. sativus* L., although it affects all cucurbits. Bacterial wilt disease has also been reported in Africa [19].

The increasing trend of local and international movement of seedlings, cuttings and fruits enhance the risk of introducing new pathogens and vectors, where applicable into new areas. Changing climate conditions can contribute to a more successful spread of pathogens and establishment of such organisms in areas previously unfavourable to their existence. A holistic disease management approach which requires the use of cultural, mechanical, biological and chemical methods is needful to mitigate the negative impact of diseases, insect and weed pests on productivity, hence, the importance of this review on the current and emerging pests and diseases of *C. sativus* L. in Africa.

#### **2. Viruses of cucumber**

#### **2.1 Genus:** *Potyvirus*

The genus *Potyvirus* is one of the largest genera of plant viruses infecting several economically important cucurbits worldwide [20]. Potyviruses are non-enveloped flexuous filamentous viruses of about 680–900 nm long and 11–15 nm wide, harbouring a monopartite genome consisting of a single positive-sense RNA covalently linked to a viral protein genome at one end and a polyadenyl tail at the other end [11, 21]. Typical symptoms induced by *potyviruses* include mosaics on leaves, leaf distortion, leaf reduction, fruit deformation, yellowing and wilting [22]. The major

**181**

*Current and Emerging Pests and Diseases of Cucumber (*Cucumis sativus *L.) in Africa*

*potyviruses* that affect *C. sativus* L. in Africa include *Zucchini yellow mosaic virus* (ZYMV), *Watermelon mosaic virus* (WMV), *Moroccan watermelon mosaic virus*

*Zucchini yellow mosaic virus* (ZYMV) naturally infects cucurbits, mostly cultivated species, a few ornamentals and weeds. There are reports of ZYMV in South Africa, Egypt, Nigeria, Sudan, Tunisia, Mali, Madagascar and Morocco [23, 24]. ZYMV is spread by aphids in a non-persistent manner [22]. Among 26 aphid species capable of transmitting ZYMV, *Aphis gossypii* Glover, *Aphis craccivora* Linnaeus, *Macrosiphum euphorbiae* Thomas and *Myzus persicae* Sulz have been identified as more efficient vectors. Mechanical and seed transmission have also been reported [25]. Symptoms produced on infected plants include vein clearing, yellow mosaic, leaf deformation with blisters, misshaped fruits and stunted growth (**Table 1**).

Planting of available resistant/tolerant varieties to ZYMV is advisable. Growing taller non-susceptible companion crop, cross- protection to control severe ZYMV isolate and spraying of insecticides are also good measures against the virus.

*Watermelon mosaic virus* (WMV), formerly WMV-2 has a wide host range which include *Cucurbitaceae*, *Apiaceae*, *Chenopodiaceae*, *Fabaceae*, *Malvaceae*, *Orchidaceae* and several weeds [33, 34]. WMV has been reported in such countries as Nigeria, South Africa and Tanzania in Africa [35, 36]. About 35 species of aphids transmit WMV in a non-persistent manner although *A. gossypii*, *A. craccivora* and *Myzus persicae* are regarded as more efficient vectors. Recent studies have also established the possibility of WMV transmission through seed [37]. Symptoms of WMV is dependent on the host and the isolate and these include vein-banding, mosaic, leaf

Eradication of weeds and alternative host helps to reduce the incidence of WMV.

*Moroccan watermelon mosaic virus* was first reported in Morocco in the year 1972 as a strain of WMV, causing severe diseases in various cucurbits [39]. Subsequently, MWMV was discovered as a distinct potyvirus species distantly related to Papaya ringspot virus (PRSV) based on biological and serological properties [40]. The host range of MWMV is limited to members of *Cucurbitaceae* family and papaya. The geographic distribution of the virus in Africa spread through Niger, Cameroon, Nigeria, South Africa, Tunisia, Tanzania, Congo and Zimbabwe [41–43]. *A. gossypii* and *Myzus persicae* transmit the virus in a non-persistent manner. Symptoms associated with MWMV infection include mosaic, severe leaf and fruit deformation,

Close monitoring of field for timely intervention in the event of WMV disease outbreak is helpful. The use of plastic mulches also reduces insect infestation which,

*DOI: http://dx.doi.org/10.5772/intechopen.96692*

(MWMV and *Papaya ringspot virus* (PRSV).

*2.1.1 Zucchini yellow mosaic virus (ZYMV)*

*2.1.1.1 Management of ZYMV*

*2.1.2.1 Management of WMV*

otherwise would transmit the virus.

wilting and dark-green blistering [41].

*2.1.2 Watermelon mosaic virus (WMV)*

deformation, fruit discolouration and distortion [38].

*2.1.3 Moroccan watermelon mosaic virus (MWMV)*

*Current and Emerging Pests and Diseases of Cucumber (*Cucumis sativus *L.) in Africa DOI: http://dx.doi.org/10.5772/intechopen.96692*

*potyviruses* that affect *C. sativus* L. in Africa include *Zucchini yellow mosaic virus* (ZYMV), *Watermelon mosaic virus* (WMV), *Moroccan watermelon mosaic virus* (MWMV and *Papaya ringspot virus* (PRSV).
