PEST AND DISEASES OF YAM (Dioscorea spp.)

INTRODUCTION

Infestations of tubers and sometimes foliage cause poor growth. Stored tubers are particularly susceptible to attack and large numbers cause shrivelling.

Adult female scales are pinkish-brown, roughly oyster-shaped, conical, with a white patch at the tip of the cone. Younger scales are relatively more white. Crawlers are yellow.

Insects

• Defoliating caterpillars: Defoliating caterpillars eat the leaf blade and then attack the stems, sometimes causing severe damage.

• Yam moth: Pyralid moth worms attack yam tubers in lofts by mining, especially during the first four months of storage.
• Chrysomelids: Chrysomelids larvae are shiny, black and covered in mucus, and may reach 1 cm. They are defoliating and can cause major damage locally, especially during the first months of the crop, but without any real economic impact.
• Yam beetle or weevil: Damage in the form of big holes (about few cms deep) occurs in tubers and is caused by adult weevils in the fields, from the period of germination until harvest. Larvae develop on the roots of other plants (grasses) in wet areas near yam fields or directly on the yam roots.
• Coffee bean weevil: The damage is caused by the hairy yellowish larvae. The brownish-red adult, which is a good flyer makes circular perforations which yield a yellow powder. Its attacks are generally confined to areas around injuries caused by harvest or caterpillars.
• Macuna: The larvae cause significant local damage to the tubers. The adults, 20 mm long, eat the leaf blades and sometimes the tubers as well.
• Tenebrionidae: The larvae and the adults eat into the surface of the tubers and then bore large areas 5 to 10 mm deep. Despite the significant size of the insects, the damage progresses quite slowly.
• Mealybug scales: These homopterans develop mainly during storage, forming a sort of white powder near the top of the tubers. They can cause complete necrosis of sprouts preventing the use of tubers as seedlings.
• Yam scale (with carapace): This species sometimes grows until it totally covers tubers during storage in the form of small and more or less whitish scales. This does not cause a loss of volume but may cause germination to be delayed or even stopped.
• Termites: Termites can attack the tubers during storage, with the infestation possibly beginning in the field. The damage, which may be significant within a few weeks, is often difficult to detect when the colonies use only one gallery for penetration.

 

Nematodes

• Root-knot nematodes: The symptoms include the development of prominent galls on the surface of tubers. There is sometimes a proliferation of tuberous roots on these galls (hirsutism). Dioscorea alata is more sensitive to this type of nematodes than Dioscorea rotundata.
• Yam nematodes: This type of nematodes causes small cracks on the surface of tubers, with the proximal parts (head), affected most. Under these lesions are brownish-black necrotized areas whose size depends on how long ago the damage occurred. The species Dioscorea rotundata is generally most affected.

Diseases

• Anthracnose: It is one of the most severe yam diseases, in particular among the species Dioscorea alata. It causes black necrotic spots on the leaves which develop along the veins. The stems can be affected as well as the end buds, thus stopping growth. In the case of a severe attack, the plants can be completely destroyed.

• Other foliar spots: These fungi cause more or less dark brown spots of various shapes on the leaves: surrounded by a yellow halo (Curvularia) or with concentric circles (Sclerotium). Severe attacks can kill the plant.
• Tuber wet rot: These micro-organisms cause soft wet rot on tubers. They develop during storage where there has been damage to the tuber or a hole made by an insect. The bacteria have a putrid odour from a close distance.

Virus

• Yam mosaic virus, Yam mosaic virus and Yam mild mosaic potyvirus, Cucumovirus, Dioscorea bacilliform virus, Potexvirus Dioscorea latent virus: Dioscorea rotundata often more susceptible than Dioscorea alata. Vectors include aphids and mealybugs.
• Internal brown spots: Symptoms appear as brown nodules in the tubers and it results in a decline in the quality of the tubers.

 Information provided by https://coleacp.org

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