MSc Huynh Kim Ngoc

Cassava mosaic disease was first discovered in the late 19th century (1894) in Tanzania, causing damage in many cassava-growing African countries such as Tanzania, Uganda, Nigeria etc,... In Asia: Cambodia, Thailand, Myanmar… The disease is difficult to control and greatly affects productivity. Recently, many cassava growing areas in Vietnam such as Tay Ninh, Binh Phuoc, Gia Lai, Phu Yen, Binh Dinh have been recorded. The disease has appeared on a large area and seriously affected productivity.

Agent:

Cassava Mosaic Disease (CMD) is caused by a virus and is transmitted through two routes: cuttings and (whitefly): Besimia tabaci.

Symptom: 

Symptoms vary depending on the virus strain, growing environment and susceptibility of the variety, but disease symptoms are common and easy to identify. Diseased leaves have mosaic symptoms: leaves discolored, yellow, white, light, dark, alternate, thick leaves, small, wrinkled blades, shrinking, deformed, curved edges, may be torn, foliage growth not normally, if infected early, plants will be dwarfed, stunted, not give tubers or give low tuber yield. Disease symptoms can appear at any growth stage of the plant, if diseased cuttings are taken to plant, disease symptoms will appear soon after the plants grow, plants are stunted, do not yield, if infected late, when mature trees, similar symptoms but low yield.

The cause to the disease

Disease caused by SLCMV virus (Sri Lanka Cassava Mosaic Virus).

Mechanism of disease transmission:

The disease is spread by cuttings and by insect vectors: Whiteflies

+ Through cuttings: Because viral pathogens are present in the sap in stems, leaves, and tubers, if the diseased cuttings are used as seed for the next crop, the disease will appear as soon as the plants grow and the tubers are left in the field after harvest, will be the source of disease spread through the next crop.

+ Through whiteflies (insect vectors): When the plant is large, if the hopper sucks the sap of the diseased plant, the virus passes through the needle, enters the hopper is body, and then if the hopper stings the healthy plant, it will transmit the virus and the pathogen. It will take little time to multiply the number and the symptoms will also show up a short time later.

Biological characteristics of whitefly:

White fly has scientific name: Bemisia tabaci, belongs to the order Hemiptera, has many phenotypes, very wide host spectrum, parasitic on over 900 crops worldwide (except Antarctica), is also epidemic. important greenhouse pests and vectors for hundreds of virus species. Whitefly is considered the most important pest worldwide. In Vietnam, in addition to cassava, whitefly is commonly found on legumes such as tomatoes, African eggplants, tobacco, red peppers, cucurbits, watermelons, cucumbers, potatoes, and even on fruit trees and ornamental flowers.

Adult whiteflies are small, about 1 mm long, with equal length front and hind wings, body and wings covered with white powder. The field is heavily infected, when entering the garden, the whiteflies fly like a cloud of chalk dust. The life cycle of the whitefly is about one month, but depending on the temperature, crops and farming practices, it can last longer, sometimes up to a whole year, including 4 stages: Egg-larva having the age of 3 - Pupa (it is documented that there is no pupal stage) and Adult. Each female whitefly can lay 200 eggs in her lifetime. The hatched larvae are pale yellow, newly hatched with legs, crawl under the leaf surface, then hide in a place under the leaf surface, after molting, at the age of 2, they no longer have legs. Both larvae and adults, which live and damage the underside of leaves, can suck the sap and secrete saliva containing the virus that spreads mosaic disease, and because saliva contains many carbohydrates, it creates conditions for sooty mold to develop. Plants with mosaic disease do not die immediately, but underdeveloped, stunted, if heavily infected, the tree is exhausted and dead.

Take care of healthy plants.

- Using recommended active ingredients such as: Pymetrozine, Imidaclroprid, Dinotefuran, Buprofezin, ... such as Osago 80WG (Pymetrozin + Nintenpyram), Butyl 10WP (Buprofezin), Brimgold 200WP (Dinotefuran + Imidacloprid), these products can be used singly or mixed with PSO mineral oil such as SK Enspray 99EC for increased effectiveness. Experimental results with Binh Phuoc Sub PPD (August 2018) showed that Osago 80WG and Brimgold 300WP gave good control results to whitefly when spraying 25 days after placing cuttings, the number of infected plants in the treated lot was very low and there was a clear difference compared to the control lot similar to farmers in Phu Yen, when combining Brimgold 200WP with SK Enspray 99EC mineral oil, also noted that the sprayed fields, the cassava was free of mosaic disease compared to the non-sprayed control fields.

- Pay attention to spraying in the early morning because the leaves are still wet with dew and the whiteflies are less moving, spray a lot of water and spray evenly on the foliage, pay attention to spray carefully the underside of the leaves where the whiteflies reside and cause harm. It is necessary to alternate products to limit resistance.