The golden cyst nematode of potatoes
May 9th, 2018 | Afnan Sehar | No Comments
The nematode is found in temperate regions including Pakistan of the world causing severe losses mainly to potatoes. However, it also attack tomato, egg plant and other solanaceous vegetables. Once this nematode infests a field, it is practically impossible to get rid of it as its eggs survive in cyst in soil for more than 20 years.
The golden nematode is primarily a pest of potatoes and is referred to as a species of potato cyst nematode. In the larval stage, it bores into the roots of host plants and feeds on their juices. Because the above ground damage is not visible during the early stages of infestation, the pest can remain undetected for years. The first sign of a golden nematode infestation is poor plant growth in one or more areas of a potato field. As an infestation builds, the damaged area increases and eventually the entire field display poor plant growth. Large numbers of the nematodes cause wilting, stunted growth, poor root development, and early plant death. For many years, only one species of potato cyst nematode was recognized, Heterodera rostochiensis. However, H. rostochiensis was later separated into two species, H. pallida with white or cream-colored females, the pale potato cyst nematode (wPCN) and H. rostochiensis with golden females, the golden potato cyst or golden nematode (PCN). Eventually, these nematodes were given the genus name Globodera. Globodera sp. have round cysts whereas, Heterodera sp. produce lemon-shaped cysts. They are found in many temperate regions of the world infesting approximately 65 countries. The first detection of the pale potato cyst nematode (wPCN) in the U.S. occurred in 2006 when this pathogen was found in soil collected from a packing shed in Idaho. The detection ignited a national voluntary survey, initiated in 2007, of all potato producing states in the U.S.
1. The infested plants have shallow, bushy root system. So,they appear as patches of stunted, yellowish plants.
2. The leaves of the infested plants are smaller and yellowish, and may wilt and die (because of the poor root system).
3. The infected potato plants bear fewer and smaller tubers but there are no symptoms on the tubers.
4. The infected roots are smaller with many white or brown nematode cysts present (feeding) on their surface.
5. On heavily infested plants, the cysts of the golden nematode can often be seen on the roots with the naked eye. Cysts are white to golden to dark brown in color, shiny, round, and are attached to the root by a short stalk.
The pathogen (Globodera rostochiensis, G.pallida)
The adult males are worms-like while the females are spherical with a projecting neck and head. The 1st and 2nd stage larvae (juveniles) are worm-like and develop inside while they are alive and feeding, are white cysts. However, later these cysts die and return brown consisting of the old body wall i.e. the leathery skin of the female nematode. These dead, brown cysts contain large number of the over-wintering eggs of the cyst nematode.
The golden nematode cyst is actually the swollen body of a dead female nematode. Each cyst contains up to 500 nematode eggs that hatch when they are near the roots of a host plant. After hatching, young (juvenile) nematodes, also called larvae, enter the plant’s roots where they feed on plant nutrients and grow to maturity. Male nematodes then leave the root. Females extrude from the roots but remain attached and at this stage appear as tiny, white, swollen objects. They are fertilized by the male nematodes in the soil, which then die. After fertilization, the females produce eggs and die, forming a hard wall around the eggs with their bodies, resembling “cyst” like structures attached to roots. Cysts are white at first, but gradually darken to a golden yellow or dark brown color. The pale cyst nematode has a similar life cycle; however, the female remains creamy white in color as it matures. Cysts detach from the roots and can survive up to 20 years or longer in soil.
The nematode overwinters in egg in dead, brown cysts. These cysts are dispersed by, water, tubers etc. The eggs hatch in spring; the second stage infective larvae come out, directly enter the roots of susceptible host, and become sedentary. The nematode gets established (invasion) in the cortex, endodermis and pericycle etc. The nematodes feed with the help of their stylets. They secrete enzymes that liquefy cell contents to be withdrawn by their stylet. They also secrete hormones such as IAA and Cytokine is which stimulate cell enlargement and cell division. Few cells around nematode head get enlarged. This enlargement happens because of two reasons (i) No cell wall is laid down around the newly formed cells.
(ii) The already existing walls around cells are dissolved. So, the protoplasm’s of several cells join together to make giant cell-like bodies called Syncytia. These syncytic affect the growth of plants by inhibiting/disturbing the growth of xylem and phloem. The body cavity of the enlarged, adult, spherical female nematode gets filled with eggs (colonization). The increasing body volume (of the female nematode) crushes the host cells, split open the root surface and gets almost entirely on eggs in it. These eggs may hatch to release the infestive 2nd stage larvae which cause new infections or may overwinter and hatch in spring.
This article is collectively authored by Afnan Sehar1 and Muhammad Nazim*2.1Department of Plant Pathology, University of Poonch Rawalakot, Azad Kashmir, Pakistan. 2 Department of Agronomy, Muhammad Nawaz Sharif, University of Agriculture Multan, Pakistan.
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