**6.2 Pests and disease management in onion (***Allium cepa***) and garlic (***Allium sativum***)**

Nutrient-deficient and poorly irrigated soils, low technology adoption rates coupled with pests and diseases have curtailed onion and garlic cultivation in Nigeria. Ewuziem and Alleluyanatha [116] highlighted purple blotch (*Alternaria porri*), black mold diseases (*Aspergillus niger*), neck and bulb rot (*Botrytis allii*), Onion twister (*Colletotrichum cingulata*), downy mildew (*Peronospora destructor*) pink rot (*Pyrenochaeta terrestris*) and bulb rot incited by *Fusarium oxysporium* as fungal diseases of economic importance affecting onion and garlic cultivation in northern Nigeria.

The biggest threats to making profit on onion or garlic cultivation in northern Nigeria is insect pests, majorly thrips. *Thrips tabaci* (Thysanoptera: Thripidae) constitute a major threat to onion which is capable of reducing bulb yield if onion is planted late. Other minor insect pests are *Zonocerus variegatus* and *Spodoptera exigua*. Late onions can harbor about 600 thrips in a plant, with the largest population being harbored by the third youngest leaf, notwithstanding the age of the crop [117]. Thrips are also controlled with the use of Lambda-cyhalothrin, neem formulation (2–3 ml/l). Malathion (2 teaspoon/l), Diazinoon 50 WP (1 table spoon/4 l), Bayfidan (triadimenol) 20 EC (2 teaspoon/4) or Dimethoate (0.05%).

Report by Kebbi Agricultural and Rural Development Authority (KARDA) in Kebbi State of Nigeria stated that a major limitation to profitable cultivation of onions is the incidence of a disease locally called 'Danzazzalau'. The devastating effect of this disease on onion productivity necessitated the study reported and concluded that *Fusarium equiseti* was responsible for the disease. *Fusarium equiseti* living in the soil affect the roots, stem plate and fleshy leaf bases of the onion plant.

*Nigeria Root Vegetables: Production, Utilization, Breeding, Biotechnology and Constraints DOI: http://dx.doi.org/10.5772/intechopen.106861*

#### *6.2.1 Purple blotch of onion*

Pattern of variation for resistance to purple blotch (*Alternaria porri*) of onions (*A. cepa* L.) in North western Nigeria was also reported by [118]. According to the report, five cultivars of onion: Ori local, Kaharda, Sokoto, Red Creole and Koumassa were selected based on genetic backgrounds diversity in respect of resistance to *Alternaria porri (Ellis.) Cif.* In a complete diallel cross of some cultivars, 25 F1s generated and their parents evaluated in a yield trial at Sokoto and Talata Mafara in Zamfara State both of Nigeria, with 31.20%, 30.58% and 5.42% disease incidence observed as phenotypic, genotypic and environmental coefficients of variability, respectively [119].

### **6.3 Garlic (***A. sativum***)**

Emechebe et al. [118] evaluated the insecticidal properties of garlic aqueous extract on beans (*Phaseolus valgaris*) and maize (*Zea mays*) pests at different concentrations and concluded that there was relationship between extracts and mortality of *Sitophilus zea mays* and *Callosobruchs maculatus*. The report of the study carried out by [120] revealed that garlic (*A. sativum*) at 5% aqueous extract concentration exhibited antifungal potential when tested against the mycelial growth of southern blight pathogen (*Sclerotium rolfsii*) of tomato. Percentage inhibition was reported to be 77% at 5% concentration in vitro and disease severity was as low as 2.7 in vivo.

#### **6.4 Pests and disease management in carrot**

Carrot is affected by a variety of pests and diseases [121]. Reducing of plant vigor and growth are part of the effects of nematodes on crops. In nematode affected fields, some plants will be observed to be heavily infested while others will not, resulting in uneven crop maturity or reduction in the quality of the produce [122, 123]. Root knot nematodes (*Meloidogyne species*) causes general reduction in plant vigor likewise severe distortions and root swelling thereby reducing the marketability of root crops like carrots [124]. Flea beetle, white flies (*Daccus* sp.), aphids, cutworms, and horn worms are among insects that destroy carrot plants [125]. The insects can be controlled by planting resistant varieties and judicious use of short acting pesticides like malathion and carbaryl (Sevin). Calcium deficiency causes blossom end rot which can be treated by addition of lime to soil or spraying the leaves with calcium solution. This disorder can also be contolled by maintaining adequate soil moisture [122, 123]. Hill et al. [122, 123] concluded that in most locations, diseases severely restrict carrot cultivation. In Nigeria and other African countries, small holder carrot cultivation has experienced a great increase both as food and cash crops in recent years, and effective management of pests and diseases is essential for sustained carrot production. Wittwer [126] highlighted major fungal diseases of carrot as *Alternaria* and *Cercospora* leaf spots or blights, leaf mold, Fusarium wilt, target spot or early blight some of which are soil-borne. Plants are affected at any developmental stage by Septoria leaf spot [127]. Powdery mildew caused by *Oidium lycopersicum* is another important fungal disease. According to [128] scouting for disease and rogueing infected plants once they are observed is very important. Soil-borne diseases of carrot according to [129] are bacterial soft rot, cavity spot, cottony rot, crown rot, phytophthora, root die-back, root knot nematode and southern blight [126]. Carrot is considered as being capable of producing higher yields and returns to vegetable farmers in some part of the country

after the rainy season [130], which calls for effective crop protection to maximize farmers'profit.
