*5.2.1.1 Management of Powdery mildew*

Planting resistant varieties, application of fungicides to delay infection and reduce disease incidence are common measures to manage powdery mildew. Additionally, use of non- hazardous biopesticide such as cinnamon oil, effective phytosanitation and regular weeding have also been recommended.

#### **5.3 Genus:** *Alternaria*

Species in the genus *Alternaria* have worldwide distribution because they are capable of existing in a variety of habitats, which explains their commonness and abundance. Members of the genus *Alternaria* are ascomycete fungi with major plant pathogens [82]. Lack of suitable hosts and unfavourable environment have been identified as some of the probable reasons for absence of *Alternaria* spp. in any environment [83]. The key distinguishing feature of the genus *Alternaria* is the production of dark coloured conidia with longitudinal and transverse septa. The spores are airborne and found in soil, water and on surfaces.

#### *5.3.1 Alternaria leaf blight of Cucumber*

Alternaria leaf blight of cucumber is caused *by Corynespora casiicola*. Although, cucumber is reported to be more susceptible to Alternaria leaf blight disease among the cucurbits, there are reports of wide host range distribution for the disease and evidences of infection abound in several other crops and weeds. Transmission of infection is carried out through wind over long distance and by splashing of water from diseased plants to previously unaffected ones over short distance. Lesions tend to appear first on older leaves in form of small circular spots. The spots are brown with light center which form dark concentric rings as they enlarge. Lesions formed on lower leaf surface tend not to be focused [84]. Fruit infection starts out as sunken brown spots and may later develop dark powdery appearance. The infection may also begin at the growing end while the entire fruit eventually turns brown. Spots on stems and petioles elongates more to distinguish the disease from other diseases such as angular leaf spot and anthracnose. In recent times, target spot has emerged with alarming severity and incidences on agriculturally important crops such as cotton (*Gossypium hirsutum*), soybean (*Glycine max*) and tomato (*Solanum lycopersicum*) in the Southeastern United States [85], causing losses of between 5–40%. In Africa, there seems to be scanty report of the disease on cucumber but it has been reported on such crops as tomato and tobacco in Nigeria [86], on potatoes in South Africa [87] and as a biocontrol agent of weed in Egypt [88]. *Corynespora casiicola* which was considered as a minor issue to production of major crops such as cotton, soybean is now causing epidemics in US by expanding its host range to previously non-host crops, it is therefore of importance considering it an emerging fungal disease which needs more attention in Africa.

**189**

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

Alternaria disease of cucumber can be effectively managed by using resistant varieties for planting and practising crop rotation with non-cucurbitaceous crops. Also, destruction of volunteer alternate hosts, application of commercially recommended fungicides and adoption of good sanitation practices should be encouraged.

Members of the genus *Fusarium* are ubiquitous saprophytes which can be isolated from debris, roots, stems and seeds of a wide variety of plants [89]. *Fusarium oxysporum* is one of the important phytopathogen causing Fusarium wilt disease in more than a hundred species of plants [90]. *Fusarium* wilt of cucurbits is caused by *Fusarium oxysporum* f. sp. *niveum* which affects watermelon, *Fusarium oxysporum* f. sp. *melonis* which affects muskmelon and *Fusarium oxysporum* f. sp. *cucumerinum*

Yield loss due to the infection of Cucumber Fusarium wilt disease has been estimated to be between 10–30%. Poor quality of harvested product also results causing huge economic losses [91]. *Fusarium* spp. cause wilt and root rot of affected plants. Invasion by the fungi starts from the root of the plant and progresses into the stems. Damping-off is the symptom of early infection which develops due to lower stem infections. Wilting generally starts on the older leaves and proceeds to the young ones. Wilt symptoms develop on one or few lateral vines in the beginning, while other branches remain apparently unaffected. However, under high inoculum conditions or in highly susceptible host species, the whole plant may wilt and die within a short time. Older plants may experience vascular browning, gummosis, and tylosis in xylem vessels [84]. Transmission of infection over short distances occur by the movement of infested soil, while the spread over long distances occur by using infected equipment and planting propagules. The disease may invade the fruits through the stem end.

Treatment of seed with chemical fungicides and planting resistant varieties are measures that can be used to manage the disease. Hot water treatment of seed at 52 °C for 30 minutes and crop rotation with distantly related crop is also helpful.

Members of the genus *Pythium* are common worldwide [92]. Many species in the genus *Pythium* are important soil-borne pathogens which affect economically important crops such as cereals and vegetables [93]. Several other *P*ythium spp. are saprophytic, with some enhancing plant growth and displaying potential as a biocontrol agent [94]. About 8 *Pythium* species have been identified in Africa, *P. aphanidematum* being the commonest. Pythium-induced damping-off and root

Damping-off caused by *Pythium* spp. constitute major constraint to cucumber production worldwide [96]. *Pythium* spp. were associated with wilt and root rot

disease of cucurbits cause huge losses, sometimes as much as 100% [95].

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

**5.4 Genus:** *Fusarium*

which affects cucumber.

*5.4.1 Fusarium wilt of Cucumber*

*5.4.1.1 Management of Fusarium wilt*

**5.5 Genus:** *Pythium*

*5.5.1 Damping-off in cucumber*

*5.3.1.1 Management of Alternaria disease of Cucumber*
