Important Crop Diseases Caused by Mycoplasma, Nematodes | Botany Optional for UPSC PDF Download

Plant Disease Symptoms caused by Plant Viruses

Plant viruses often reveal their presence through distinct symptoms exhibited by the infected host plants. These symptoms can be influenced by factors such as the plant's age, nutritional status, and environmental conditions.
Here are some common symptoms associated with plant virus infections:

1. Chlorosis: The presence of plant viruses can lead to chlorosis, where the green parts of the plant lose chlorophyll and develop yellowish spots. This symptom often creates a mosaic-like pattern on the green tissue, giving rise to diseases known as "mosaic" diseases.

• Mosaic diseases are typically caused by viruses that are mechanically transmitted and may have aphid vectors in their natural transmission cycle.
• In most cases, mosaic diseases do not disrupt flowering or affect bud dormancy.

2. Yellows: In severe cases of virus infection, chlorophyll can completely disappear from the host tissue, resulting in yellowing, bronzing, or reddening of affected organs. These symptoms are collectively referred to as "yellows."

• Viruses causing yellows are generally transmitted by leaf-hoppers and are relatively sensitive to heat treatment.

3. Vein Clearing and Vein Banding: Virus infections can lead to the disappearance of chlorophyll along the veins of leaves, known as "vein clearing." Conversely, when chlorophyll around the veins disappears, it is referred to as "vein banding."

4. Necrosis: Necrotic spots develop as a result of the death and subsequent drying of plant tissue. These brownish spots are known as "necrotic spots" or "lesions."

5. Ring Spots: Some viruses cause the appearance of chlorotic or necrotic rings on leaves, fruits, and stems. Most ring spot-causing viruses are transmitted by nematodes.

Other Symptoms: Plant viruses can induce various other symptoms, including:

• Bunchy top
• Galls
• Hypertrophy (abnormal enlargement of plant organs)
• Atrophy (wasting or reduction in size of plant organs)
• Rolling (leaf roll of potato)
• Curling (leaf curl of potato)
• Crinkling of plants

These symptoms serve as visual indicators of plant virus infections and play a crucial role in their identification and management. Understanding these symptoms can aid in diagnosing and addressing viral diseases in crops and ornamental plants.

Plant Disease Symptoms caused by Mycoplasma

Mycoplasma-like organisms (MLOs) can induce various symptoms in plants, with some of the most important ones being "little leaf," "yellows," "grassy-shoot," "big bud," and "phyllody."
These symptoms are indicative of mycoplasma plant diseases:

1. Little Leaf: Little leaf refers to a reduction in the size of the leaf lamina, which is a characteristic symptom of mycoplasma infections. This symptom is commonly observed in plants like brinjals (eggplants), tomatoes, and cotton.

2. Yellows: Similar to the symptom in virus-infected plants, mycoplasma infections can lead to the complete disappearance of chlorophyll from the host tissue, resulting in organs turning yellow. This symptom is referred to as "yellows" and is a characteristic feature of mycoplasma-induced plant diseases.

3. Grassy-Shoot: Grassy-shoot is a term used to describe the appearance of affected shoots, which take on a grassy or dwarf-like appearance. This symptom is observed in plants such as sugarcane and rice affected by mycoplasma infections.

4. Big Bud: In some cases, mycoplasma infections can cause fruit-bearing shoots to become straight, thickened, and malformed, forming thick dark green structures. An example of this symptom is "big bud" in tomato plants caused by mycoplasma.

5. Phyllody: Phyllody occurs when the apical shoots of affected plants are transformed into leafy structures. This symptom is observed in various plants, such as sesamum and san hemp, and is associated with mycoplasma infections.

These symptoms serve as diagnostic indicators of mycoplasma-induced diseases in plants and can vary depending on the plant species and the specific strain of MLO involved. Identifying these symptoms is crucial for the diagnosis and management of mycoplasma-related plant diseases.

Plant Disease Symptoms caused by Nematodes

• The symptoms caused by nematodes appear on the roots as well as on the above ground parts of plants. The symptoms on the roots appear as hypertrophy, root knots or root galls, root lesions, excessive root branching, injured root tips and root rots.
•  The root symptoms are usually accompanied by the malformation and blistering of the above ground parts of the plants, twisting or distortion of leaves and stems, and abnormal development of the floral parts. Certain nematodes attack grains forming galls full of nematodes in place of seed.
• Root knots, cockle, hard smut, false ergot and brown root rot are common symptoms. These infestations bring about deficient nutrition resulting in partial or complete wilting in advanced cases.

Nematode Diseases and Their Control

Introduction

In a basic plant pathology text, it is impossible to cover all aspects of plant nematology. There are hundreds of species of plant-parasitic nematodes, and students seeking information on this diverse group of parasites should consult one of several nematology textbooks. This chapter concentrates on the most widespread and economically important nematodes in Australia and New Zealand.

Sedentary Endoparasites

Root-Knot Nematodes (Meloidogyne spp.)

• Root-knot nematodes, Meloidogyne spp., are the world's most damaging nematode genus. They are widely distributed in the tropics and subtropics and are common in temperate regions where summers are warm to hot. Severe infestations cause total crop loss, while yield losses of 5-20% occur in some crops despite routine use of nematicides. There are more than 40 species of root-knot nematodes, but worldwide, 95% of the damage is caused by just four species: M. arenaria, M. hapla, M. incognita, and M. javanica. These species attack more than 2,000 plant species, including most crop plants. Some crops commonly infected by root-knot nematodes in Australia and New Zealand are listed in Table.
• The disease cycle begins when second-stage juveniles hatch from eggs, move through the soil, and invade roots near the tips. These juveniles affect the differentiation of the plant's cells near their heads, so that multinucleate giant cells are formed through multiple mitoses without cytokinesis. The juveniles then become sedentary and start feeding on these giant cells, thus establishing a specialized host-parasite relationship. Developing nematodes eventually lose their worm-like shape and molt three times to become adults. The pear-shaped adult females are embedded in gall tissue and can be observed by carefully teasing galls apart under a stereo microscope. In most Meloidogyne spp., males are rare, and reproduction occurs by parthenogenesis. Each mature female lays hundreds of eggs in an egg mass outside her body. These eggs are protected from desiccation by a gelatinous material and hatch in warm, moist soils to continue the life cycle. The length of the life cycle is temperature-dependent, but at temperatures of 24-28°C, a generation takes 4-6 weeks. Continued infection of galled tissue by second and later generations of nematodes causes the large galls sometimes seen on plants such as tomatoes at the end of the growing season.

Crops Commonly Infested by Root-Knot Nematodes in Australia

• Horticultural Crops: almond, grape, kiwi fruit, nectarine, passionfruit, pawpaw, peach, plum, banana, ginger, pineapple, strawberry, aloe vera.
• Vegetable Crops: bean (mung, French, navy), beetroot, capsicum, carrot, celery, cucurbits (cucumber, melon, pumpkin), eggplant, lettuce, okra, onion, potato, sweet potato, tomato.
• Field Crops: clover, cowpea, kenaf, lucerne, lupin, pigeon pea, peanut, soybean, sugarcane, tea, tobacco.
• Ornamental Crops: carnation, chrysanthemum, dahlia, gerbera, gladioli, protea, ozothamnus (riceflower), rose.

The presence of nematodes in the root stimulates the surrounding tissues to enlarge and produce the galls, which are the typical symptom of infection by root-knot nematode. Galling restricts root volume and hinders the normal translocation of water and nutrients within the plant, so that plants exhibit above-ground symptoms of stunting, wilting, and chlorosis. Damage caused by the nematode also predisposes plants to attack by other soil-borne pathogens, particularly fungi and bacteria. The end result is a loss in yield and a reduction in the quality and marketability of plant products that are produced underground (e.g., tubers and rhizomes).

Fumigants such as ethylene dibromide and methyl bromide and chemicals that liberate methyl isothiocyanate have been widely used for control of root-knot nematodes, particularly in high-value horticultural, vegetable, and ornamental crops. Since the availability of many of these broad-spectrum biocides is declining, they have been replaced in some instances by organophosphate and carbamate nematicides. These materials are acetylcholinesterase inhibitors and therefore affect processes under the control of the nervous system (e.g., host finding, feeding, egg hatch). Since the chemicals tend to be nematostatic rather than nematicidal, nematode activity resumes when the concentration of chemical declines below a critical level. Thus, control is generally maintained for only a relatively short period.

Because there are health and environmental risks associated with the use of most nematicides, considerable effort is being devoted to the development of alternative control strategies. Rootstocks with resistance to root-knot nematodes have been successfully used in the grape and stonefruit industries for many years, and in the long term, resistance is likely to be the most convenient and economically feasible method of controlling root-knot nematodes in other cropping systems. Resistance genes are available in wild plant species, but at present, there are few commercially available crop cultivars with resistance to one or more species of root-knot nematode. Development of resistant cultivars and rootstocks has been slow because of the genetic diversity of Meloidogyne, problems in making interspecific and intergeneric crosses of some plant species, and difficulties in preventing the transfer of deleterious genes closely linked to the resistance genes. Recently, tobacco with transgenic resistance to Meloidogyne has been produced. The resistance involves a gene that destroys the giant cells produced by the developing nematode, thereby preventing nematodes from obtaining nutrients from these cells.

Crop rotation also has potential for use in managing root-knot nematode, but its value is limited by the specificity of resistance genes. However, it is not always possible to use species identification to determine host range, as populations with different host ranges can occur within one Meloidogyne species. Grasses are generally more resistant to Meloidogyne than non-grasses and are often useful in rotation with Meloidogyne-susceptible crops. Rotation crops must have a high level of resistance; otherwise, sufficient nematodes may carry over to damage the next susceptible crop. Some cultivars of maize, signal grass (Brachiaria decumbens), and forage sorghum show good resistance to most species and races of Meloidogyne in Australia.

Cereal Cyst Nematode (Heterodera avenae)

• Heterodera avenae is distributed worldwide on cereals and most grasses. It is one of the major causes of yield loss of wheat in the winter rainfall areas of southern Australia and is also a problem on barley and oats.
• In early autumn, with the onset of rain and falling temperatures in the southern parts of Australia, second-stage juveniles hatch from eggs that have survived over summer within cysts. The juveniles migrate to the roots of the seedling crop, enter behind the root tip, and then become sedentary, feeding on specialized cells called syncytia, which consist of multinucleate transfer cells formed by extensive cell wall dissolution among contiguous cells. Towards the end of winter, the adult female swells, ruptures the root cortex, and protrudes from the roots so that it is visible to the naked eye. Females are fertilized by worm-like males and then lay eggs retained within the body. As the females age and die, their body walls harden and darken to form mature, brown cysts that protect eggs during summer. There is therefore only one generation per year.
  
• In the field, damage caused by H. avenae typically appears as patches of poor growth similar to those caused by nitrogen deficiency or water stress. Plants are stunted and yellowish. When examined closely, the roots have small swellings or knots, with many side roots protruding from these swellings, giving the roots a brush-like appearance. Soil tends to adhere closely to infected roots, making them difficult to wash clean.
• Juveniles hatch in response to low temperature and moisture, so in the absence of a host plant, eggs hatch but juveniles cannot find a suitable feeding site, leading to their death without reproducing. Nematode populations decline at the rate of annual hatching (70-90% per annum) in the absence of a host plant. Rotations are therefore effective in controlling H. avenae. Generally, two years free of host crops (susceptible cultivars of wheat, barley, oats, rye, and triticale) and weeds (particularly wild oats and annual ryegrass) are needed to reduce nematode numbers below the economic threshold. Useful rotation crops include resistant cereal cultivars, lupins, leguminous pasture, beans, and peas.
• Resistant cultivars can reduce nematode reproduction and maintain low nematode populations. Genes for resistance to the Australian pathotype of H. avenae are available in wheat, barley, oats, rye, and triticale. Since there are a number of sources of resistance and only one pathotype of H. avenae has been found in Australia, the potential for using resistance genes to overcome any change in nematode pathotype is good. Tolerance (the capacity of cultivars to withstand attack from nematodes with minimal yield loss) is also a useful attribute and is often used in combination with resistance.

Potato Cyst Nematodes (Globodera rostochiensis and G. pallida)

• For many years, Australia was the only continent free of potato cyst nematode. However, since 1986 it has been found in Western Australia and Victoria. The nematode probably originated with the potato in South America. Although its main host is potato, it can also reproduce on other solanaceous crops such as tomato and eggplant.
• There are two species of potato cyst nematode, golden (Globodera rostochiensis) and pallid (G. pallida). Both species occur in New Zealand, but evidence to date suggests that only the golden nematode occurs in Australia. Since races can occur within species, race identification is important when making recommendations for control with resistant cultivars.
• The life cycle is similar to that of H. avenae, with one generation being completed on each crop. Between crops, eggs survive within cysts in soil. When a potato plant is growing, substances exuded by the roots stimulate the eggs to hatch. Each egg contains a second-stage juvenile which hatches, moves from the cyst into the soil, and penetrates a host root just behind the root tip. The juvenile establishes a permanent feeding site in the root and develops to become an adult.
• After reaching the adult stage, males leave the root and move through the soil to find females. Females remain in the root, expanding and eventually rupturing it, remaining attached by the head and neck only. After fertilization, the female produces 300 to 500 eggs which she retains within her body. The female dies with the root, but the cuticle hardens and tans, forming a protective cyst for the eggs.
• Potato cyst nematode is one of the most serious pests of potatoes. Low populations of the nematode are often not noticed because above-ground symptoms are not obvious. However, as the number of nematodes increases, plants become stunted, leaves are smaller and yellowish, and plants dry off early. Yields may be reduced by as much as 80%, mainly due to the production of smaller tubers.
• The potato cyst nematode is such a serious pest that quarantine controls are strictly enforced in most countries. Once introduced, hygiene measures and local quarantine procedures can be adopted to slow the rate of spread. The potato cyst nematode is most commonly introduced in soil adhering to tubers, machinery, and vehicles, or in contaminated soil associated with root and bulb crops imported from nematode-infested regions.
• It may be many years from the time potato cyst nematode is introduced to its detection by visible symptoms. In this time, the nematode can spread throughout the crop and to other crops and properties. To reduce the chances of this happening, a susceptible potato crop should be grown only once every four years. In the other three years, other non-solanaceous crops or resistant potatoes should be grown. Once the nematode becomes endemic to a region, cultivar resistance is the most feasible control option. Cultivar Atlantic is resistant to race 1 of the golden nematode, present in restricted areas in Western Australia and Victoria.

Sedentary Semi-Endoparasites

Citrus Nematode (Tylenchulus semipenetrans)

• This nematode is by far the most important nematode pest of citrus. It occurs in all citrus-producing regions of the world and limits production under a wide range of environments. It is also economically important on grapes in areas where citrus and grapes are grown together.
• The first-stage juvenile develops within the egg and molts once before hatching as a second-stage juvenile. Once in the soil, the juvenile survives on stored food reserves until a suitable citrus root is located and a specialized feeding site consisting of several nurse cells is established in the root cortex. Once feeding commences, the nematode starts to grow, completing three additional molts. The posterior portion of the female body remains outside the root and swells, eventually assuming a characteristic kidney shape. The mature female produces a gelatinous material that covers the entire body of the nematode and contains several hundred eggs. One generation of the nematode normally takes 6-8 weeks at soil temperatures of 24-26°C.
  
• Reproduction is bisexual but may also be parthenogenetic, with unfertilized females laying eggs that will develop into juveniles of both sexes. The male juvenile does not feed and will develop into an adult within 7-10 days. Soil populations consist of both newly hatched juveniles and males.
• Citrus nematode is associated with a slow decline of established citrus trees. Nematode-free trees planted into fumigated soil and infested trees planted into clean soil can tolerate the nematode for about 10 years before nematodes begin to cause economic damage. The extent of decline in mature citrus trees is related to their vigor and tolerance to the nematode and to the degree of infection. Slow decline, as the name implies, develops gradually on mature trees, beginning with the production of smaller and fewer fruits. Generally, environmental conditions that stress the tree (i.e., infertile soil, marginal salinity, alkaline soils, extreme fluctuations in soil moisture and temperature) exacerbate the effects of the citrus nematode. The symptoms of slow decline are often nondescript and difficult to diagnose, so the presence of citrus nematode is best confirmed by microscope analyses of soil and root samples. Heavily infected roots often appear encrusted with soil particles that are not easily washed off due to soil adhering to the gelatinous material excreted by the female during egg production. There are usually fewer and shorter feeder roots on infected plants compared with uninfected plants.
• The most serious and rapid effects of the nematode occur when young trees are planted in old citrus soil heavily infested with the nematode, a condition referred to as the citrus replant problem. Young trees develop slowly in such situations, delaying production.
• Sanitation is important in preventing nematode infestation of new or fumigated land. Nursery trees should be free of nematodes. Once citrus nematode becomes established, it is virtually impossible to eliminate as it can survive for at least 7 years after an old, infested orchard is removed. The nematodes may also survive for extended periods on other hosts, such as persimmon, olive, and grape. Most management systems for citrus nematode are designed to minimize environmental stress on the tree so that its economic impact is minimized.
• Resistant or tolerant rootstocks provide protection from some biotypes of citrus nematode. Some cultivars of Poncirus trifoliata are highly resistant to citrus nematode, and F1 hybrids with Citrus spp. (e.g., Carrizo and Troyer citrange) are useful against some biotypes. The resistance is characterized by a hypersensitive response to nematode feeding and the subsequent formation of wound periderm.
• Chemical control with organophosphate and carbamate nematicides is the primary management strategy in some countries. However, the cost of nematicides and the need to reapply them regularly limits their usefulness.

Migratory Endoparasites

Lesion Nematodes (Pratylenchus spp.)

• Lesion nematodes are widely distributed, and their economic importance is frequently underestimated. In Australia, some of the most important problems involve P. brachyurus (many crops), P. thornei and P. neglectus (cereals), P. penetrans (pome fruits), and P. zeae (sugarcane and other grasses). Most species have a wide host range.
• Lesion nematodes remain migratory throughout their life cycle, moving within roots and from root to root. Males, females, and all juvenile stages are infective. When nematodes enter the root cortex, they move within and between cells, depositing their eggs in root tissue. Eggs then hatch to continue the life cycle, which takes about 27 days at 27-30°C.
• Pratylenchus usually destroys the outer cortical tissue of roots, but nematodes may reach the vascular tissues in some hosts. As the nematodes feed, they destroy cells, so extensive lesions develop when large numbers of nematodes are present. When the destruction has proceeded beyond a certain point, the nematodes migrate from damaged roots and move into more favorable tissues. Elongate, narrow, dark lesions are characteristic of roots infested by Pratylenchus, but they are not diagnostic. Formation of lesions is often followed by root rotting due to invasion of fungi and bacteria. Pratylenchus may also be involved in disease complexes (e.g., wilts of solanaceous crops caused by Pratylenchus and Verticillium).
• Control procedures for Pratylenchus tend to vary with the crop involved. In horticultural crops such as apple, stonefruit, or grape, where Pratylenchus spp. cause severe damage when trees or vines are replanted, nematicides are often used. On broad-acre crops, lesion nematode problems are often not recognized. Crop rotation is frequently unsuccessful because of the wide host ranges of most species. Genetic resistance has not yet been exploited as few sources of resistance have been identified.

Burrowing Nematode (Radopholus similis)

• Radopholus similis is an important pathogen of banana throughout the tropics and subtropics, where it causes "toppling disease." In Australia, R. similis is found throughout the main banana-growing areas of Queensland and New South Wales.
• The life cycle of the burrowing nematode consists of the egg, four juvenile stages, and the adult. The juveniles and females penetrate roots, parasitize host tissue, and cause decay. Nematodes feed directly on the cell cytoplasm, so the nucleus disintegrates and the cell wall ruptures, forming a cavity. Nematodes continue to enlarge these cavities by feeding and tunneling in the cortex. Males have a degenerate stylet, so they do not feed and cannot enter roots. However, they can be observed inside roots after developing juveniles in the tissues undergo the final molt into males. More than one generation can occur inside the root, but usually, as the root deteriorates, the nematodes migrate to the soil in search of other roots. Females lay 4-5 eggs per day. Both males and females are required for reproduction, with the egg-to-egg cycle being completed in 20-25 days at temperatures ranging from 24-32°C.
  
• The above-ground signs of the burrowing nematode are leaf chlorosis, dwarfing, a thin pseudostem, small bunches, and premature lodging of plants. Dark red lesions appear on the cortical or outer part of the root as a result of nematode infection. The cortex later turns black as the nematodes multiply and other organisms invade the tissues. Healthy roots are bone-white. Nematodes do not invade the central region of the root (i.e., the stele) unless it is colonized by secondary invaders (e.g., fungi). Nematode infestation can be easily detected by making a longitudinal cut along the root. Heavily infected roots have numerous depressed and opened lesions from which the cortical tissue sloughs off easily. The lesion eventually girdles the root, causing the roots to break. Plants with short necrotic roots cannot support the weight of the bunch or the stress of moderate to heavy winds. As a result, the plant falls over or becomes uprooted. Other symptoms include reduction in bunch weight and increased time to bunching.
• R. similis is currently controlled by routine use of chemical nematicides. However, because the nematode is disseminated with infected corms, the selection, cleaning, and treatment of planting material is an important control procedure. Corms free of necrotic parts are selected and should be properly peeled so that necrotic tissue is discarded. Peeled corms can then be dipped in hot water (55°C for 15 minutes) and left to dry for 24 hours before planting. The use of disease-free, tissue-cultured plantlets is now encouraged.

Ectoparasitic Nematodes

• These nematodes are widely distributed, and most soil samples contain at least some ectoparasitic species. Dagger, stubby, and needle nematodes are the most economically important ectoparasites.
• Stubby root nematodes (Trichodorus and Paratrichodorus) have a wide host range and cause symptoms of stunting, chlorosis, and reduced yield. Roots of infested plants may be short, stubby, and slightly swollen. This group of nematodes also transmits some plant viruses.
• Dagger and needle nematodes (Xiphinema, Longidorus, and Paralongidorus) cause stunting and swelling of roots and poor above-ground growth. They tend to be most important on perennial crops, but a serious disease of rice in north Queensland is caused by P. australis. These nematodes are also vectors of some plant viruses.
• Many other ectoparasitic species cause economic damage, but they tend to be important only in specific crops or situations. High numbers of nematodes must usually be present before crop losses occur. The nematodes involved include stunt nematodes (e.g., Tylenchorhynchus, Merlinius), ring nematodes (e.g., Criconema, Criconemella, Macroposthonia), sheath nematodes (e.g., Hemicycliophora, Colbranium), pin nematodes (e.g., Paratylenchus), and spiral nematodes (e.g., Rotylenchus, Helicotylenchus, Hoplolaimus, Scutellonema).
• Ectoparasitic nematodes remain outside the host throughout their life cycles or penetrate with only a small portion of their body. They have a relatively simple life cycle as eggs are laid in soil and development through all juvenile stages to the adult occurs in soil. All stages of the life cycle can feed on roots. The length of the life cycle varies considerably, from a few weeks in most species to months or even years for some dagger and needle nematodes.
• Since ectoparasitic nematodes feed by inserting only their stylet into roots, many species cause no obvious symptoms. Often thousands of nematodes must be present before damage becomes apparent. The more pathogenic species tend to feed on root tips, causing root stunting, thickening, or galling of root tips and a general reduction in the number of feeder roots.
  

Above-Ground Parasites

Stem and Bulb Nematode (Ditylenchus dipsaci)

• Ditylenchus dipsaci is widespread in cool temperate regions of the world. There are at least 20 morphologically indistinguishable biological races. Races attacking onions, daffodils, faba beans, and lucerne, clover, and field peas have been known in New South Wales, Victoria, Western Australia, and South Australia for many years. An oat-attacking race is present throughout South Australian cereal regions.
• All developmental stages of the nematode invade the stems and leaves of plants, entering the host through the stomata. The nematode colonizes parenchymal tissues where it feeds and reproduces. Each female may lay 200-500 eggs. Nematode feeding and movement cause large cavities in the infected tissues. Because the nematodes require a film of water on the outside of the stem to move to the penetration site, nematode infestations are favored by wet conditions and mild temperatures (15-20°C). When the environment is favorable, the life cycle takes 19-23 days. Under unsuitable conditions, the nematode survives as a quiescent fourth-stage juvenile in seed, bulbs, and tubers. In this dehydrated state, nematodes form a white, cottony mass sometimes called "eelworm wool." They can survive in this state for decades.
• The characteristic symptom of infection by D. dipsaci is deformed leaves and bulbs. Infected oat plants tend to be stunted, swollen, and distorted with a proliferation of tillers. Severely affected plants are killed, resulting in bare patches in infested crops. Internodes of legume hosts tend to be shortened, resulting in stunted plants. At the infection site, stems become cracked and rotten. In crops such as faba bean and field pea, the nematode causes necrosis of the stem base and distortion of leaves, stems, buds, and pods. Fourth-stage juveniles accumulate at the necrotic stem base and inside the seeds. On lucerne and clovers, the nematode distorts buds and causes chlorosis, swelling, and loss of leaves. Distortions and swellings of leaves and bulbs are the most common symptoms induced on bulbous plants. At late infection stages, the plant may die and rot due to secondary invasion by pathogenic bacteria and fungi.
• Physical control methods such as hot water treatment of bulbs and tubers are effective in preventing dispersal of the nematode in planting material. Chemical nematicides may control the nematode but are rarely cost-effective. Even though the nematode can survive in a metabolically inactive state for many years in a protected environment, under field conditions of wetting and drying, the survival time is much less. Crop rotation can therefore be useful. Effective management practices include a 2-3 year rotation with non-host crops, good weed control, wide row spacing, and replacement of overhead irrigation with drip irrigation. These strategies prevent nematode build-up in soil and spread among plants. Resistance is likely to be the most successful form of control in the long term, and resistant cultivars are available for some crops.

Foliar, Leaf, or Bud Nematodes (Aphelenchoides spp.)

• Aphelenchoides spp. are widely distributed. Economically important species in Australia include A. besseyi (strawberry and rice), A. fragariae (ferns and succulent plants), A. ritzemabosi (many Asteraceae, including chrysanthemum), and A. composticola (mushroom). Although often referred to as foliar, leaf, or bud nematodes, common names such as strawberry crimp nematode (A. besseyi), white tip of rice (A. besseyi), or chrysanthemum foliar nematode (A. ritzemabosi) are also used.
• Nematodes in infected plant tissue or soil climb the plant in moist, humid conditions and feed on buds or leaves. Some species feed ectoparasitically, others endoparasitically. Generally, the nematodes are tissue-surface feeders. They migrate, feed, and reproduce in water films and enter leaf intercellular spaces via stomata to feed on the cells of the spongy mesophyll. They find suitable conditions for survival in the protected environment of buds and around the growing point. Their life cycle is 10-14 days, and they can survive in dry leaves for several months.
• Feeding causes necrosis of damaged leaf tissue, producing blotches on leaves. The form and pattern of the blotches closely follows the leaf anatomy and venation, so lesions are often sharply delimited by veins. Feeding within buds may kill the growing point or bud, resulting in distorted and abnormal growth.
• Removal and destruction of old foliage at the end of the season will lessen the risk of spread. Use of clean planting material or hot water treatment of infested planting material is also a practical method of control. In crops where leaf wetness is caused by irrigation rather than rainfall, a change in irrigation methods or timing may create conditions less conducive to the nematodes.

Annual Ryegrass Toxicity (Anguina funesta)

• Annual ryegrass toxicity (ARGT) is caused by a bacterium, Rathayibacter toxicus, which is spread into the plant by the nematode Anguina funesta. The nematode and bacterium are spread throughout the wheat/sheep belts of South Australia and Western Australia, where ARGT can cause deaths of sheep, cattle, and horses. The nematode, but not the bacterium, has been found in the Wimmera region of Victoria. Related nematodes and bacteria have also been found in association with toxicity of Agrostis avenacea and Polypogon monspeliensis to sheep and cattle in western New South Wales and southeastern South Australia. 
• Control of ARGT involves reducing annual ryegrass in paddocks. This means using selective herbicides in crops so that ryegrass does not survive. In pastures, ryegrass is controlled by burning or with herbicides. Monitoring of grazing paddocks by examining seed will give an early warning of developing toxicity.
• Acknowledgment: The figure on page 514 has been provided by the Department of Primary Industries, Queensland, from their book Queensland Agricultural Journal (September-October, 1986) published by the DPI Queensland.