PLANT DISEASES CAUSED BY NEMATODES

Nematodes are lower invertebrate animals and are perhaps the most numerous multicellular animals on the earth. They are generally free-living in marine, freshwater or soil environments, but a large number of species are parasitic to different kinds of plants and animals. The parasitic species are of considerable agricultural, clinical and veterinary importance as pests of plants and parasites of man and livestock respectively. Plant parasitic nematodes (PPN) are eel worms which are essentially aquatic and spend a greater part of their life cycle in the soil. They feed on the surface or the peripheral layers of the root or enter the root and feed from within with the help of a feeding structure called the stylet. Infected plants in general exhibit stunting, yellowing of leaves, wilting and reduced yield, in addition to several below-ground symptoms. Plant parasitic nematodes lay eggs singly or in masses either in the soil or within plant tissues. Most PPN have four larval stages between the egg and adult, with intervening moults. A life cycle from egg to egg can be completed within 3-4 weeks under optimum environmental conditions; temperature being the key factor in determining the duration of the life cycle.This topic deals with how nematodes cause diseases in plants, the symptoms they induce, their lifecycle an interaction with other disease causing agents, their life cycle, dispersal and survival. The above components are discussed in relationship to the control and management of diseases.

Lecture objectives

At the end of this lecture of this lecture the student should be able to

1)      Distinguish plant nematodes from other types of plant pathogens.

2)      Explain the basic procedures of diagnosing plant nematodes

3)      Explain the various measures of managing plant nematodes.

4)      Train other farmers on diagnosis and management of plant nematodes

BIOLOGY OF PLANT PARASITIC NEMATODE

Life cycle

The life histories of most PPN are in general quite similar in that all have four larval stages. Eggs may be laid singly or stuck together in masses in a gelatinous matrix secreted by the females. Some females (Heterodera spp.) die and the cuticle tan to form cysts. Many Heterodera spp. also produce a proportion of their eggs in a gelatinous matrix (egg mass) attached to the cyst. In root-knot nematodes (RKN), all the eggs are laid in an egg sac which may be buried partially within the host-derived root gall which Meloidogyne spp. induce during feeding. Egg masses are also produced by the semi-endoparasitic nematodes such as Rotylenchulus reniformis. Egg sacs and cysts serve to protect the eggs from desiccation and natural enemies.  The juvenile within the egg develops to adult through four moults, the first moult normally occurring within the egg. The egg develops into a first stage juvenile (J1). The juvenile coils several times within the egg shell and lies still. The J1 grows in size and undergoes the first moult within the egg and then hatches as a J2. The J2 is fully developed except that it lacks reproductive organs and is small in size.

The J2 undergoes a second moult and becomes a J3 and the J3 undergoes a third moult to become a J4. The J4 undergoes a fourth moult and differentiates into adult females and males and then matures. A life cycle from egg to egg can be completed within 3-4 weeks under optimum environmental conditions.

How nematodes cause disease

Plant parasitic nematodes depend solely on plants for growth and reproduction (obligate parasite). They obtain nutrients from the cytoplasm of living root, stem and leaf cells for development, growth and survival. Nematodes possess a hollow and a protrusible feeding structure called the stylet. The nematodes use this structure to pierce and penetrate the cell wall of a plant cell, inject saliva secretions and withdraw and ingest nutrients from the cytoplasm. Nematodes that enter root tissue also use their stylet to cut openings and/or inject secretions to dissolve or weaken the cell wall or middle lamella. In general, all plant parasitic nematodes damage plants by direct mechanical injury using the stylet during penetration and/or by secretion of enzymes into the plant cells while the nematode is feeding.

Infective stages

While the J2 is the infective stage in root-knot, cyst, seed gal nematodes, all stages of ectoparasites and most migratory endoparasites are infective.  In Rotylenchulus spp., the immature female is the infective stage while in D. Dipsaci the J4 is the infective stage.

Nematode induced symptoms

Symptoms may vary according to nematode parasitic habits, host nematode relationships, and other factors such as host age and physiological conditions. They include above and below- ground symptoms.

Above – ground symptoms

Symptoms associated with root nematodes are a direct result of the impaired ability of root systems to take up water and nutrients and thus are essentially similar to symptoms of any root damage that interfere with the physical support and water and nutrient absorption systems. They are thus often similar to mineral deficiencies, inadequate or excessive water supply and generally poor soils. Symptoms are more pronounced if the plants are already affected by other adverse conditions or are attacked by other pathogens. Plants growing under highly favourable conditions may be heavily attacked by nematodes but show few above ground symptoms.

The most universal above ground symptoms are

  1. Stunting ât the reduction of growth rate, reduction in amount of foliage and progressive death (die-back) of plants.
  2. Chlorosis (yellowing of leaves), poor yield, early senescence, premature dropping of fruits and flowers, fruit malformation. The stunted and chlorotic plants are distributed in circular to oval areas of variable size in the field but patches of damaged plants may be elongated if infested soil is moved in the direction of cultivation
  3. Wilting due to the effect on the functions of roots.
  4. Other above ground symptoms are associated with specific nematode species. For example:
  • Leaves with dark green spots, angular or cuneiform in shape, with interveinal discoloration and necrosis are associated with Aphelenchoides ritzemabosi on chrysanthemum leaves while twisting and white tips of leaves of rice are associated with Aphelenchoides besseyi.
  • Yellowing and collapse of palm trees followed by a rapid death and a red necrosis in the vascular bundles on the stem forming a red ring in coconut and oil palm is due to infection by Bursaphelenchus cocophilus.
  • Galls in stems, leaves and seeds of cereals and grasses are caused by Anguina spp.
  • Toppling of banana plants especially during fruit bearing is due to Radopholus similis.
  •  Twisting of leaves and raised yellow lesions on stems and leaves on onions are by Ditylenchus dipsaci),
  • Twisted panicles and empty grains by Ditylenchus angustus on rice.
  • Distorted apical growth and crimpling of leaves and inflorescence (Aphelenchoides besseyi and Aphelenchoides fragariae in strawberry) and so forth.

 

Below-ground symptoms

  1. Reduction of root system especially the secondary feeder roots
  2. Abnormal development of roots
  • Overall root galling (Meloidogyne spp. and Nacobbus aberrans)
  • Lesions/ulcerations. Sharply demarcated necroses in different layers of the plant tissues. This results from reactions of phenolic substances in the plants to the secretions discharged by nematodes. Roots with longitudinal necrotic areas are typical of Pratylenchus spp., Radopholus spp.; Hirschmaniella spp. infection
  • Dry rots usually develop from infestation of fleshy parts of the plants (tubers, root vegetables, stolons) eg by Ditylenchus dipsaci on onions, D. destructor on potatoes, R. similis on banana rhizomes. The nematodes cause dry rots in association with secondary invading microorganisms.
  • Excessive branching of secondary roots (M. hapla, Pratylenchus spp. N. aberrans. Localized proliferation of lateral roots (Some Meloidogyne spp and Heterodera spp).  Parasitism of young roots stimulates formation of lateral roots. The lateral roots are also infected and the entire root system becomes dendroid and reticulate in appearance. In Heterodera spp. this causes the “root-beard” disease
  • Swollen, hooked root tip galls (Subanguina spp, Xiphinema spp, Meloidogyne graminicola).
  • Retardation of growth on the root tip. The root system is dwarfed and thickened in appearance (Trichodorus, Longidorus, Xiphinema spp,).
  • Roots ending in rounded galls (Longidorus spp. and Hemicycliophora spp).
  • Stubby roots, suppression of root growth (Trichodorus spp and Paratrichodorus)

 

Nematode dispersal

Nematodes can be dispersed actively or passively.

Active dispersal

Nematodes do not move very far or very quickly by their own locomotory power in the soil. Active nematode migration mostly occurs in the rhizosphere as they are attracted to root exudates. Meloidogyne spp., for example, are attracted to an area just behind the root tip while others such as Pratylenchus are attracted to the root-tips and sometimes further back. The root tip is the region of high metabolic activity from which numerous substances diffuse, some of which act as attractants (gibberellic acid, glutamic acids, tyrosine, amino acids and carbon dioxide), some as repellants and some neither. The best known example of active migration above ground is by Aphelenchoides spp. (e.g. A. ritzemabosi) which moves up the wet external plant parts and then invades the leaves of the host plant.

Passive dispersal

  • Dispersal by water

Water is a frequent means of passive dispersal. This may include surface run-off such as overland flow, streams, rivers, irrigation canals, percolation and interflow. Infiltration and percolation of water accounts for some downward nematode dispersal but the distance varies with soil properties and precipitation. Dispersal of Radopholus similis, for example, is aided by percolation. Interflow is lateral underground movement of water where percolation water is forced laterally when it comes in contact with an impervious soil layer. Above ground nematodes can be splashed to plants by falling rain or overhead irrigation.

  • Dispersal by wind

Wind blowing on bare soils or on low-growing plants or young plants can disperse nematodes e.g. Heterodera schachtii, G. rostochiensis, Criconemoides, Helicotylenchus, Meloidogyne, Pratylenchus, Tylenchorhynchus spp. etc.

  • Phoretic dispersal

This includes the involvement of another animal to aid dispersal. Insects are important in dispersing nematodes that attack the aerial parts of plants. Nematodes can pass through the digestive tract of animals and remain infectious in some instances, being dependent on the mobility and the speed of the vector and the survival capacity of the nematode. For example, the palm weevil, Rhynchophorus palmarum disperses Bursaphelenchus cocophilus (red-ring of coconut palm). The J3 is deposited onto the palm tree when the weevil is ovipositing her eggs. Bursaphelenchus xylophilus (pine wilt) is transmitted by Monochamus alternatus; a cerambycid beetle. In this instance, the nematode’s 4th stage juvenile is carried on the outside of the beetle under the elytra and in the trachea and deposited into the pine as the beetle feeds.

  • Planting materials

Nematodes can spread through planting materials such as seeds, vegetative propagating materials (tubers, corms, bulbs), seedlings and rootstocks.  Nematodes spread this way can lead to serious losses in the mature crop or in subsequent crops if nematode build -up is not checked. Seedlings/transplants: Nematodes that are associated with seedlings in nurseries are transferred to the field during transplanting.

Dispersal by other means

Man’s activities also aid in the dispersal of nematodes. E.g. transport of infected plant materials between different countries (international dispersal) or parts of the countries (local spread) or by farm implements between fields and cultivations within fields.

Management of plant parasitic nematodes

There are several methods commonly used to control plant-parasitic nematodes. These methods can be divided in to three main types: biological control, cultural control and chemical control. The most practical form of biological control is the use of nematode-resistant plants. In this control method, plant breeders cross natural nematode resistance genes into cultivated plant species to improve their resistance to nematodes. The benefit of this method is that it is a very inexpensive way for growers to control their nematode problems. The main disadvantage is that it takes years to screen for resistant plant varieties and more time to breed resistance traits into commercial varieties. Further complications are that natural sources of nematode resistance do not exist for all cultivated species and some species of nematodes are able to grow on resistant plants. However, when “good” resistant plants are available, they are an effective method of nematode control. Crop rotation with a non-host plant is a very effective method to limit nematode growth. Typically, a cropping system is devised that selects plants that nematodes can and cannot grow on. These plants are grown in alternate years and the problematic nematode population decreases dramatically, below damage threshold levels, in the years that the non-host is grown. This can be an effective method if a producer has the choice of several different crops that can be grown and if the problematic nematode does not have a broad host range or survive in the soil in a cryptobiotic state for long periods of time.

For the over 50 years now nematodes have been effectively controlled using chemical nematicides. These are inexpensive chemicals that effectively kill nematodes in soil. There are two types of nematicides, soil fumigants (gas) and non-fumigants (liquid or solid). Soil fumigants became popular because they did not rely on alternative host crops for rotation; they drastically reduced nematode populations in the soil, and were cost effective for most crops. Most fumigant nematicides have been banned by the EPA as environmental toxins with the exception of 1,3 dichloropropene (Telone II), chloropicrin (tear gas), and dazomet (Basamid). The multipurpose soil fumigant methyl bromide also provides excellent reduction of soil nematode populations, but methyl bromide was largely discontinued in 2005. Non-fumigant nematicides such as fenamiphos (Nemacur) and aldicarb (Temik) are based upon the same kinds of active ingredients as many insecticides (i.e. nerve poisons) and can be applied in liquid or granular formulations. While non-fumigant nematicides reduce nematode populations, their effectiveness is not as consistent as that of fumigant nematicides. The EPA is also restricting the use of non-fumigant nematicides. Since nematicides are expensive to develop, new ones are rarely released on the market today. While nematicides are effective in controlling nematodes, they are only practical for use on high-value crops.

Summary

Correct diagnosis of plant nematodes should be carried out to ensure their effective and timely management. Plant nematodes are not typically controlled using just one method but instead they are managed using a    combination of methods in an integrated pest management system.

 

References

Kaya, Harry K. et al. (1993). “An Overview of Insect-Parasitic and Entomopathogenic Nematodes”. In Bedding, R.A. Nematodes and the Biological Control of Insect Pests. Csiro Publishing

Nyle C. Brady & Ray R. Weil (2009). Elements of the Nature and Properties of Soils (3rd Edition). Prentice Hall