*2.1.1 Zucchini yellow mosaic virus (ZYMV)*

*Cucumber Economic Values and Its Cultivation and Breeding*

(*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

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

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

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

losses. Scab affects aboveground plant parts, including the fruits.

**180**

farmers [13].

diseases of *C. sativus* L. in Africa.

**2. Viruses of cucumber**

**2.1 Genus:** *Potyvirus*

*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**).

### *2.1.1.1 Management of ZYMV*

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.

#### *2.1.2 Watermelon mosaic virus (WMV)*

*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 deformation, fruit discolouration and distortion [38].

#### *2.1.2.1 Management of WMV*

Eradication of weeds and alternative host helps to reduce the incidence of WMV. 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, otherwise would transmit the virus.

#### *2.1.3 Moroccan watermelon mosaic virus (MWMV)*

*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, wilting and dark-green blistering [41].


**183**

**Causal**

Virus Bacteria

Bacterial wilt

[26]

Fungi

Damping-off

BPYV

**Disease**

**Picture**

**Reference**

[31]

Fungi

Fusarium wilt

**Causal**

**Disease**

**Picture**

**Reference**

[26]

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

*Abbreviations: ZYMV: Zucchini yellow mosaic virus; WMV: Watermelon mosaic virus; MWMV: Moroccan watermelon mosaic virus; PRSV: Papaya ringspot virus; CMV: Cucumber mosaic virus; BPYV: Beet pseudo-yellows virus.*

**Table 1.**

*Symptom expression of some cucumber diseases.*

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

[32]

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

**Table 1.**

 *Symptom expression of some cucumber diseases.*

*Cucumber Economic Values and Its Cultivation and Breeding*

**182**

**Causal**

Virus Virus Virus Virus Virus

CMV

[26]

Fungi

Alternaria leaf blight

[26]

PRSV

[30]

Fungi

Powdery mildew

[26]

MWMV

[28]

Nematode

Downy mildew

[29]

WMV

[26]

Bacteria

Root Knot

[26]

ZYMV

**Disease**

**Picture**

**Reference**

[26]

Bacteria

Angular leaf spot

**Causal**

**Disease**

**Picture**

**Reference**

[27]

## *2.1.3.1 Management of MWMV*

Use resistant varieties for planting and avoid planting close to old cucurbit fields. Phyto-sanitation, use of virus-free planting materials and pesticide for control of the vectors have also been employed in the management of MWMV.

#### *2.1.4 Papaya ringspot virus (PRSV)*

*Papaya ringspot virus* (PRSV) has great economic importance for cucurbit and papaya cultivation worldwide. The designated biotype for cucurbits is PRSV-W. The virus has been reported on cucurbits in Tunisia [44], Egypt [45], Morocco [46] and South Africa [47]. Early season infection may lead to poor fruit set while late season infection may result to blotchy fruit. The virus can be transmitted through movement of farm workers and machinery from one place to another and by aphids. At initial stage, symptoms can appear as vein clearing of leaves followed by the development of dark-green mosaics. In cucumber, leaves are distorted along the margins.

#### *2.1.4.1 Management of PRSV*

Management of infection on cucurbits can be achieved by using resistant varieties for planting. Insecticide applications may also reduce aphid numbers in the field. Other practices may include the use of mulches and elimination of volunteer weed hosts.

#### **2.2 Genus:** *Cucumovirus*

Viruses in this genus have wide host range, infecting over 1200 plant species worldwide [48]. Virions are icosahedral particles of approximately 29 nm in diameter, made of subunits of single capsid protein numbered 180 [49, 50]. The genus consists of three linear positive-sense single stranded RNA molecules. The major *Cucumovirus* infecting *C. sativus* L. in Africa is *Cucumber mosaic virus* (CMV).

#### *2.2.1 Cucumber mosaic virus (CMV)*

The first report of cucurbits-infecting virus was obtained on *Cucumber mosaic virus* in 1916. CMV is of great importance in temperate and tropical regions of the world [51]. Report of CMV infection on cucurbits in Africa were established in Tanzania and South Africa [36]. Over 80 species of aphids in more than 30 genera transmit CMV in a non-persistent manner but *Aphis gossypii* and *Myzus persicae* are the most efficient [52, 53]. Rapid spread of the virus has been attributed to the attraction of CMV-infected plants to aphids [54]. Transmission through seed, parasitic weeds such as dodder *Cuscuta* spp. and mechanically have also been established [50, 55, 56]. CMV causes typical mosaic symptoms in cucumber which include mosaic on leaves or fruit, stunted growth, deformed fruit, yellow spot and wilting (**Table 1**).

#### *2.2.1.1 Management of CMV*

The use of certified seed for planting, regular weeding and disinfection of hands and farm tools are effective ways to manage CMV. Aphid population should also be kept under control through regular spraying with insecticide.

**185**

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

The genus *Crinivirus* comprises of members with cross-banded flexuous particle, five gene nodule and infection that is restricted to the vascular tissues [57]. A typical member of this genus infecting cucurbits in Africa is *Beet pseudo-*

*Beet pseudo-yellows virus* has a broad host range including cucurbits, ornamentals and weeds. BPYV particles are approximately 12 nm wide and 1500–1800 nm long [58]. BPVY has been reported in South Africa [11]. The genome is made up of two linear positive sense, single stranded RNA of about 7.6 to 8 kb, both required for infectivity [57]. Typical BPVY symptoms may be confused with physiological disorders. Symptoms appear first on older leaves as yellow spots which develop into yellow blotchy raised areas between veins, which remain green. Subsequently, younger leaves become affected but the fruit

Prevention of incidence of whitefly infestation, practising crop rotation and regular weeding are recommended ways of managing BPVY. Elimination of inter-

The genus *Erwinia* comprises rod-shaped bacteria that are plant-pathogenic

Bacterial wilt, caused by *Erwinia tracheiphila* and vectored by striped cucumber beetle (*Acalymma vittatum* F.), is one of the serious diseases threatening natural and wild cucurbit crops. Cucumber is one of the most susceptible to the disease [60]. Bacterial wilt remains a major disease of cucurbits in South Africa [61]. Transmission occurs when an infected beetle creates wound during the course of feeding on a plant and defecates therein. The bacterium moves by free water into the xylem, which spreads and multiplies it. Disease symptom appears first on younger plants and these include discolouration of stem tissues and wilting of some

Disease management relies on controlling cucumber beetles, mainly through insecticide applications. Practicing crop rotation and planting cultivars that are less

attractive or susceptible to striped cucumber beetles are also helpful.

cropping of old and young plants and good sanitation are also effective.

and plant-associated [59]. Bacteria in this group are related to *Escherichia coli*, *Shigella*, *Salmonella* and *Yersinia*. A typical member of this genus which affects *C. sativus* L*.* is *E. tracheiphila* which causes bacterial wilt of cucurbits in

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

*2.3.1 Beet pseudo-yellows virus (BPYV)*

**2.3 Genus:** *Crinivirus*

*yellows virus*.

remain intact.

**3.1 Genus:** *Erwinia*

general.

*2.3.1.1 Management of BPYV*

**3. Bacterial diseases of cucumber**

*3.1.1 Bacterial wilt of Cucurbits*

or all the parts of the affected plant [62].

*3.1.1.1 Management of Bacterial wilt disease of cucurbits*

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