The following points highlight the three major types of insect transmitted diseases and their management. The types are: 1. Insect Transmission of Bacterial Diseases 2. Insect Transmission of Phytoplasma Diseases 3. Insect Transmission of Spiroplasma Diseases.
Contents
Type # 1. Insect Transmission of Bacterial Diseases:
Approximately two hundred species of bacteria have been proved pathogenic to plants. Some of these are extremely destructive, while others are of minor economic importance.
Most bacterial (prokaryotic) diseases of plants do not require insects as vectors, relying instead on rain, wind, soil, seed dispersal, or other means of transport and entry to plants. However, insect vectors do contribute to the spread of some bacterial pathogens of plants.
Most plant pathogenic bacteria are small, rod-shaped, unicellular organisms averaging less than 1 micron in width and less than 2 micron in length. They are unable to penetrate the uninjured cuticle of plants and therefore must gain entrance through natural openings or through injuries.
The wounds made by insects are utilized for this purpose by many bacterial plant pathogens. Bacteria and bacterial diseases have many other characteristics that make them well suited to insect transmission.
Role of Insects in Bacterial Diseases of Plants:
Most plant diseases caused by plant pathogenic bacteria which are produced within or between plant cells, escape to the surface of their host plants as droplets or masses of sticky exudates (ooze). The bacteria exudates are released through cracks or wounds in the infected area, or through natural openings such as stomata, nectarthodes, hydathodes, and sometimes through lenticells, present in the infected area.
Such bacteria are then likely to stick on the legs and bodies of all sorts of insects, such as flies, aphids, ants, beetles, whiteflies, etc., that land on the plant and come in contact with the bacterial exudates. Many of these insects are actually attracted by the sugars contained in the bacterial exudates and feed on it, thereby further smearing their body and mouthparts with the bacteria-containing exudates.
When such bacteria-smeared insects move to other parts of the plant or to other susceptible host plants, they carry on their body numerous bacteria. If the insects happen to land on a fresh wound or on an open natural opening, and there is enough moisture on the plant surface, the bacteria may multiply, move into the plant, and begin a new infection.
The type of insect transmission of bacteria is probably quite common and widespread among bacterial diseases of plants, but it is passive and haphazard, depending a great deal on the availability of wounds or moisture on the plant surface.
A further point that has been made is that insects which, whether above or below ground, wound the host plant organs (roots, shoots, leaves, fruit, etc.) by feeding or by ovipositing in them, increase the probability of transmission of plant pathogenic bacteria.
This occurs because such insects place the bacteria, with their mouthparts or the ovipositor, in or around wounded plant cells, where they are surrounded by a suspension of nutrients (plant cell sap) in the absence of active host defences and where they can multiply rapidly and subsequently infect adjacent healthy tissues.
Few examples of plant diseases in which bacteria are spread by insects passively as described above are bacterial bean blights, fire blight of apple and pear, citrus canker, cotton boll rot, crown gall, bacterial spot and canker of stone fruits, etc.
In several bacterial diseases, however, the causal bacterium has developed a special symbiotic relationship with one or a few specific types of insects and depends a great deal on these insects for its spread from infected to healthy host plants. Some of the better known bacterium- insect associations are described briefly below.
a. Bacterial Soft Rots and Dipterous Insects:
The relation of dipterous insects to the bacterial soft rot of plants constitutes one of the most interesting to the bacterial soft rot of plants constitutes one of the most interesting association between insects and plant diseases. The relationship was first discovered in a study of the seed-corn maggot as a factor in the development of the diseases in potato.
Bacterial soft rot primarily caused by the bacterium Erwinia carotovora pv. carotovora, to some extent by Pseudomonas chrysanthemi, and, occasionally, by species of Bacillus and Clostridium. The species E. c. pv. carotovora causes the vast majority of soft rots on fleshy plant organs of any type (leaves, blossoms, fruit, stems, or roots), especially in storage and under cover or in plastic bags are subject to bacterial soft rots.
The soft rot bacteria enter the plant organ through a wound, sometimes in the field but more commonly during storage, and there they multiply rapidly, secrete enzymes that separate the cells from each other and macerate the plant cell walls, which cause the tissues to become soft and to rot.
The soft rotting bacteria survive in infected fleshy organs in storage and in the field, in plant debris and to some extent in the soil and in the pupae of several insects.
The insect larvae become contaminated with the bacteria as they feed in, or crawl about on, infected seed pieces; they also carry the bacteria to healthy plants and there they deposit them into wounds they create.
Even when the plants or storage organs are resistant to soft rot bacteria and can normally stop the advance of the bacteria by developing a barrier of cork layers, the maggots destroy the cork layers as fast as they are formed and the soft rot continues to spread.
Some other related flies, for example, the bean seed maggot Delia florilega, Drosophila busckii (Diptera: Drosophilidae), and probably others, seem to have analogous relationship to the soft rot of potato and other fleshy organs. It has also been shown that several other flies have similar relationships with soft rot bacteria and the host plants on which they prefer to feed.
Such relationships, for example, exist between the cabbage maggot, Delia radicium and soft rot in the Brassicaceae; the onion maggot, Delia antiqua (Meigen), the onion black fly, Tritoxa flexa (Diptera: Otitidae), the seed corn maggot, and the onion bulb fly, Eumerus strigatus (Diptera: Syrphidae) and the soft rot of onion.
The exact relationship between soft rot in each host and each specific insect found to possibly be involved in the transmission of soft rot bacteria from one organ or plant to another is not clear.
There is little doubt, however, that insect transmission of soft rot bacteria does occur, that insects help introduce the bacteria into wounds they open, and that the presence of insects in soft- rotting tissues inhibits the defense reaction of the plants against the bacteria.
The insects also, by carrying the soft rot bacteria internally in their bodies, help the bacteria survive adverse environmental conditions. On the other hand, the bacteria seem to help their insect vectors by preparing for them a more nutritive substrate through partial maceration of the host plant tissues.
(i) Potato Bacterial Soft Rot and Black Leg:
Soft rot (Erwinia carotovora var. carotovora, E. chrysanthemi):
Symptoms: Soft rot symptoms include rotted tissues that are wet, cream to tan in colour, and soft. Rot begins on the tuber surface and progresses inward. Infected tissues are sharply delineated from healthy tissue by dark brown or black margins.
Shallow necrotic spots on the tubers result from infections through lenticels. Rotting tissue is usually odourless in the early stages of decay, but develops a foul odour as secondary organisms invade infected tissue. Soft rot can also infect wounded stems and roots.
Black Leg (Erwinia carotovora):
Symptoms:
Plants with blackleg are stunted and have a stiff, erect growth habit. Foliage becomes chlorotic and the leaflets tend to roll upward at the margins. Plants may wilt. Stems of infected plants exhibit an inky black decay. The base of the stem is often completely rotted.
In relatively dry soil, only the pith may show blackening. Tuber symptoms for blackleg are similar to those of soft rot. The soft rot Erwinia carotovora ssp. may cause wilting but affected plants lack the characteristic inky black stem decay.
Management:
The pathogens that cause these diseases occur wherever potatoes are grown. The severity of the disease depends on seed-handling techniques, soil moisture and temperature at planting, environmental conditions; cultivar, amount of infection in the seed lot used, and external sources of the bacteria such as irrigation water and cull piles.
1. Use high quality seed.
2. Split applications of water soluble calcium applied at 1.0 to 2.0 Q/ha during bulking have been shown to reduce infection and severity of soft rot.
3. Harvest mature tubers with welt-set skins and avoid mechanical injury.
4. Avoid excessive soil moisture before harvest to reduce lenticel infection; use clean water to wash potatoes; and avoid water films on tuber surfaces during storage.
5. Postharvest curing and storage temperatures can be a critical component of soft rot management.
6. Specific temperature recommendations vary depending on the level of decay evident at packing and the market destiny of the potatoes (i.e., processing, fresh market, or long-term storage)
(ii) Onion and Garlic Soft Rot:
Symptoms:
Bacterial soft rots are characterized by softening and water soaking of one or more of the inner fleshy scales of the bulb. Affected tissue is yellow initially, turning brown as the disease progresses lengthwise in the bulb. The neck of infected bulbs may be soft when pressed. These organisms generally appear just before or at the time of harvest or in storage.
Management:
Avoid overhead irrigation once onions start to bulb (bulbing occurs about the time the bulb is twice the diameter of the neck). Harvest only after onion tops are well matured. Provide for quick drying following topping, especially if temperatures are high.
(iii) Carrot bacterial soft rot (Erwinia carotovora pv. carotovora):
Symptoms:
Bacterial soft rot appears as a soft, watery, and slimy decay of the taproot. The decay rapidly consumes the core of the carrot, often leaving the epidermis intact. A foul odor may be associated with soft rot. Aboveground symptoms include a general yellowing, wilting, and collapse of the foliage.
Management:
In the field, maintain good drainage and avoid practices that could wound roots. Avoid prolonged irrigation of mature carrots during warm months of the year. In the packinghouse, handle carrots carefully to avoid bruising and store them under cool conditions. Chlorine added to the wash water helps to eliminate the soft rot bacteria from carrot surfaces.
(iv) Bacterial Soft Rot of Rhizome and Pseudostem: Erwinia spp.:
Symptoms:
It is characterised by a massive soft odorous rot of the centre or a portion of the rhizome. The rot progresses up the pseudostem destroying the growing point and causing internal decay often with vascular discolouration. Externally, the symptoms sometimes resemble those of fusariam wilt. Yellowing and wilting of the leaves are the characteristic symptoms.
Management
1. Soil and plant drenching with bleaching powder at 2 g/1 water at an interval of 10- 15 days was found effective in controlling the disease
b. Bacterial Wilt:
(i) Bacterial Wilt of Cucumber and Muskmelon:
Bacterial wilt, caused by the bacterium Erwinia tracheiphila, is a common and destructive disease of cucurbits transmitted by beetles. The disease is most common on cucumber and muskmelon, but less damaging to squashes and pumpkins. Watermelons are apparently not affected by it. Losses from bacterial wilt vary from the premature death of occasional plants to as high as 75 percent of a crop.
Symptoms:
Expression of the disease symptoms varies with different crop species. On cucumber and melon, bacterial wilt first appears on leaves as dull green patches that rapidly increase in size. Within a day or two the wilting symptoms spread to leaves up and down the runner. In a short time these and later affected leaves on the runner turn brown, wither, and die.
The bacteria spread from the infected runner to the main stem and then to other runners. The entire plant soon wilts, shrivels, and dies. Less susceptible plants, such as squashes and pumpkins, may show a dwarfing of the vines, sometimes accompanied by excessive blossoming and branching.
Management:
1. Selection of a resistant cultivar is the most effective method of controlling bacterial wilt diseases. To control cucumber beetle grow hybrids or cultivars which have less of the “bitterness” factor in their lines. Because cucurbitacin B and cucurbitacin C, which are attractive to the beetles, are the compounds which cause the bitterness factor in cucumbers.
2. Maintain good weed control in and around the fields.
3. Remove or destroy affected plants to reduce bacterial wilt diseases and to reduce sites for beetle hibernation.
4. Vigilant sanitation through the removal of affected vines is essential for control of cucumber wilt in susceptible cultivars.
5. The most effective disease control is prompt elimination of cucumber beetles. The beetles can transmit squash mosaic virus as well as bacterial wilt and can cause severe damage by feeding.
(ii) Stewart’s Bacterial Wilt of Corn:
Stewart’s bacterial wilt of corn is caused by the bacterium Erwinia stewartii and is spread by corn flea beetles. Foliage symptoms include linear, pale green to yellow streaks that tend to follow the veins of leaves and originate from feeding marks of the corn flea beetle. These streaks soon become dry, brown and tend to be irregular and variable in size and shape.
The bacterium that causes Stewart’s bacterial wilt over winters in the guts of corn flea beetles. Adult corn flea beetles feed on corn seedlings in late spring and early summer and contaminate the feeding wounds with the bacterium. Warm winter weather conditions favour the survival of the corn flea beetle and disease development in the following spring.
Cold winters reduce beetle populations and limit disease development and spread. Stewart’s bacterial wilt can be especially destructive on some sweet corn hybrids and corn inbreeds.
(iii) Fire Blight of Apple and Pear (Erwinia amylovora):
Erwinia amylovora is a native pathogen of wild, rosaceous hosts. It was the first bacterium proven to be a pathogen of plants. Now, fire blight is an important disease of apples and pears in many parts of the world.
It has been reported that more than 200 species belonging to many insect groups, including aphids, leafhoppers, psyllids, beetles, flies, ants, honeybees, wild bees, bumblebees, and wasps, are known to transmit diseases.
As honeybees, wild bees, bumblebees, wasps, and other insects visit apple, and other flowers infected with fire blight bacteria, their mouthparts, legs, and other body parts become smeared with the bacteria in the nectar.
The insects then carry the bacteria and deposit them in the nectar of healthy flowers they visit and there the bacteria start new infections. The bacteria, however, do not survive on or in the insects for more than a few days and do not appear to over winter in association with the insects.
Symptoms: Symptoms of fire blight can be observed on all above ground tissues including blossoms, fruits, shoots, branches and limbs, and in the rootstock near the graft union on the lower trunk. Generally, symptoms of fire blight are easy to recognize and distinguishable from other diseases.
Blossom Clusters and Young Shoots:
The floral receptacle, ovary, and peduncles become water soaked and dull, grayish green in appearance. Later these tissues shrivel and turn brown to black. During periods of high humidity, small droplets of bacterial ooze form on water soaked and discoloured tissues. Droplets are initially creamy white, becoming amber tinted as they age.
Shoot Symptoms:
Tips of shoots may wilt rapidly to form a “shepherd’s crook”. Leaves on diseased shoots often show blackening along the midrib and veins before becoming fully necrotic. Numerous diseased shoots give a burnt, blighted appearance of plant.
Foliar Symptoms:
Bark on younger branch becomes darkened and water-soaked. At advanced stages, cracks will develop in the bark, and the surface will be sunken slightly. Amber-coloured bacterial ooze mixed with plant sap may be present on bark.
Fruit Symptoms:
Indeterminate, water-soaked lesions form on fruit surface and later turn brown to black. Droplets of bacterial ooze may form on lesions, usually in association with lenticels. Severely diseased fruits blacken completely and shrivel.
Root Symptoms:
The bark of infected rootstocks may show water-soaking, a purplish to black discoloration, cracking, and signs of bacterial ooze. Red-brown to black streaking may be apparent in wood just under the bark. Symptoms of root stock blight can be confused with Phytophthora collar rot.
Management of Blossom Blight:
1. In apple, avoid growing highly susceptible genotype to fire blight.
2. Maintain good weed control in and around the orchard.
3. Removal of over wintering (“holdover”) cankers is accomplished by inspecting and pruning trees during the winter.
4. Prevention of blossom infection is important in fire blight management. Effective control through pruning requires that cuts are made 20-25 cm (8 to 10 inches) below the visible end of the expanding canker and that between cuts the pruning tools are disinfested with a bleach or alcohol solution to prevent transmission.
5. Sprays of antibiotics, streptomycin or oxytetracycline, have effectively suppressed blossom infection in commercial orchards.
6. Spraying of bacterium Pseudomonas fluorescens.
7. Controlling insects such as plant bugs, psylla and avoiding the use overhead sprinklers.
c. Insect Transmission of Xylem—Inhabiting Bacteria:
Quite a few important bacterial diseases of plants, primarily trees, are caused by the fastidious bacterium Xylella fastidiosa. These bacteria inhabit the xylem of their host plants. The diseases they cause differ from the vascular wilts caused by conventional bacteria in that instead of wilt they cause infected plants to decline, some of their twigs to die back, and in some cases the whole plant to die.
The xylem-inhabiting fastidious bacteria are transmitted in nature only by xylem-feeding insects, such as sharpshooter leafhoppers (Cicadellidae: Cicadellinae) and spittlebugs (Cercopidae). Xylella bacteria seem to be distributed in tropical and semitropical areas worldwide.
Among the most important diseases caused by Xylella are Pierce’s diseases of grape and citrus variegated chlorosis. Other diseases caused by xylem-inhabiting bacteria include phony peach, plum leaf scald, almond leaf scorch, bacterial leaf scorch of coffee, oak leaf scorch, and leaf scorch diseases of oleander, pear, mulberry and some ornamentals, as well as the alfalfa dwarf disease
(i) Grape Pierce’s Disease (Xylella fastidiosa):
The occurrence of the following four symptoms in mid-to late summer indicates the presence of Pierce’s disease.
Symptoms:
1. Leaves become slightly yellow or red along margins in white and red varieties, respectively, and eventually leaf margins dry or die in concentric zones.
2. Fruit clusters shrivel or raisin.
3. Dried leaves fall leaving the petiole (leaf stem) attached to the cane.
4. Wood on new canes matures irregularly, producing patches of green, surrounded by mature brown bark. Delayed and stunted shoot growth occurs in spring following infection even in vines that did not have obvious symptoms the preceding year.
The bacterium that causes Pierce’s disease lives in the water-conducting system of plants (the xylem) and is spread from plant to plant by sap-feeding insects that feed on the xylem. Symptoms appear when a significant amount of xylem becomes blocked by the growth of the bacteria. Insect vectors for Pierce’s disease belong to the sharpshooter (Cicadellidae) and spittlebug (Cercopidae) families.
Management:
a) Insecticide treatments aimed at controlling the vector in areas adjacent to the vineyard have reduced the incidence of Pierce’s disease by reducing the numbers of sharpshooters immigrating into the vineyards in early spring.
b) During the dormant season, remove vines that have had Pierce’s symptoms for more than one year; they may be chronically infected and are unlikely to recover or continue to produce a significant crop.
c) Remove vines with extensive foliar symptoms on most canes and with tip dieback of canes even if it is the first year that symptoms have been evident.
d) Pruning a few inches above the graft union of vines with moderate foliar symptoms may eliminate Pierce’s disease and allow vigorous regrowth the following year, but symptoms will reappear in many (30-40%) or most of these severely pruned vines the second year.
d. Insect Transmission of Phloem Inhabiting Bacteria:
Under this category at least four diseases are observed which includes citrus greening, papaya bunchy top disease, cucurbit yellow vine disease and in frequent clover club leaf disease. The Citrus greening bacterium transmitted by psyllid, while papaya bunchy top and clover club leaf disease by leaf hopper vectors, and cucurbit yellow vine disease bacterium transmitted by the squash bug.
(i) Citrus Greening:
Citrus greening is a very destructive disease of all types of citrus. The disease is caused by the bacterium Liberobacter asiaticum in Asia, and L. africanum in Africa. Infected plants and vectors have been introduced into several citrus-producing countries but in most cases it was eradicated before it could become established.
The vector of the greening bacterium Diaphorina citri was introduced in Brazil in the early 1980s and in Florida in 1998.
Symptoms:
The disease first appears as a chlorosis and leaf mottling on one shoot or branch, which it has given it the name “huanglongbing”, or “yellow shoot”, in Chinese. Later on, entire trees become chlorotic as though they are suffering from zinc deficiency, their twigs die back, and the trees decline rapidly and become non-productive.
Fruit on diseased trees is small, lopsided, and does not color uniformly as it ripens but large parts of it remain green even when mature, thereby the “greening” name of the disease. Diseased fruit is also quite bitter.
Management:
Control of citrus greening requires an aggressive integrated pest management strategy:
a) Keep citrus greening disease out of bud wood supplies and nursery.
b) Remove existing sources of citrus greening within an area.
c) Replant with disease-free trees grown from clean bud wood.
d) Chemical and biological control of Asian citrus psyllid, even in areas that appear to be free of the disease.
(ii) Bunchy Top of Papaya:
Bunchy top is a devastating disease of papaya. Bunchy top of papaya is caused by a rickettsia-like phloem-limited bacterium that moves and multiplies in the phloem elements of the plant. The bunchy top bacteria are transmitted from diseased to healthy papaya plants by the leafhoppers Empoasca papaya and E. stevensi.
Symptoms:
Symptoms appear 30-45 days after inoculation. Young leaves of infected plants show mottling, then chlorosis and marginal necrosis, and become rigid. Internodes become progressively shorter, further apical growth stops, and the plant develops a “bunchy” top. Older leaves may fall off, any fruits that are set are bitter, and the entire plant may die.
(iii) Cucurbit Yellow Vine (CYV):
Cucurbit yellow vine disease is caused by a phloem-limited bacterium that has been placed in the species Serratia marcescens and its properties are still being characterized. The bacterium is most probably transmitted by insect vectors. The squash bug, Anasa tristis, is considered to be a vector of this bacterium, but its involvement in transmitting this bacterium has been questioned.
Symptoms:
Affected plants show vines with yellow leaves, the phloem of leaves and vines becomes discolored, and the leaves and vines collapse and die.
Management:
i. It is critical to control early-season adult squash bugs as they colonize the field to successfully manage this disease. Insecticide applications should target young seedlings (< 3-5 true leaves) as soon as squash bugs are present on the plants. New generation synthetic pyrethroids tend to work better. Spraying seedlings with synthetic pyrethroids should also control cucumber beetles and bacterial wilt.
ii. Early-planting of straight neck summer squash as a perimeter trap crop in the border rows of their watermelon fields to attract and control squash bugs.
Type # 2. Insect Transmission of Phytoplasma Diseases:
a. Aster Yellows:
Serious outbreaks of aster yellows can be caused by large number of migrant leafhoppers like Macrosteles fasciforns in the spring, warm weather in May and June, and adequate precipitation and soil moisture. The aster yellows pathogen infects over 300 hosts, with plant species occurring in 50 families.
Aster yellows is capable of infecting such cultivated crops as carrot, celery, cucurbits, potato, sage, tomato, echinacea, canola, flax, barley, wheat, oats, rapeseed, sunflower and faba beans.
Symptoms:
Affected plants develop general chlorosis(yellowing) and dwarfing of the whole plant, abnormal production of shoots and some times, roots, sterility of flowers, malformation of organs, and a general reduction in the quality and quantity of yield.
In carrots, symptoms known as “red top”, include increased root hairs and stunted root growth. The infected carrot will taste bitter and will appear slender and elongated. Younger foliage will appear yellow, turning to red or purple. The petioles will be dwarfed and twisted, with a dense fern-like growth of shoots.
The aster yellows pathogen exists in several strains. Aster yellows is transmitted by budding or grafting and by several leafhoppers. The phytoplasma survives in perennial ornamental, vegetable, and weed plants. The vector leafhopper acquires the phytoplasma while feeding by inserting its stylet into the phloem of infected plants and withdrawing the phytoplasma with the plant sap.
After an incubation period, when the insect feeds on healthy plants it injects the phytoplasma through the stylet into the phloem of the healthy plants, where it establishes infection and multiplies.
Infected plants usually show symptoms after 8-9 days at 25°C and 18 days at 20°C, whereas no symptoms develop at 10°C. Aster yellows phytoplasma is limited primarily to the phloem of infected plants. Some cells adjacent to the phloem first enlarge and then die.
Management:
Eradication of perennial weed hosts from the field and planting susceptible crops away from crops harbouring the pathogen help eliminate a large source of phytoplasma inoculum.
a. Weed control to reduce the presence of host plants of the vectors. Irrigation can increase the attractiveness of carrots by increasing soil moisture which maintains plant succulence.
b. Control of the leafhopper vector in the crop and on nearby weeds with insecticides as early in the season as possible helps in reducing transmission of the phytoplasma.
b. Pear Decline:
The phytoplasma organism that causes pear decline is carried by pear psylla (Psylla pyricola Forster). Psylla transmits the disease when they feed on pear foliage. The expression of the disease depends on rootstock susceptibility, tree vigor, and psylla numbers. The organism apparently does not multiply in pear trees as well as it does in pear psylla and remains infective for several weeks.
Symptoms:
Poor shoot and spur growth, dieback of shoots, upper rolling of leaves, reduced leaf and fruit size, and premature leaf drop characterize pear decline. Sudden tree collapse can result from tissue damage at the graft union on highly susceptible rootstocks such as Pyrus serotina or P. ussuriensis, but slow decline of trees is more common.
Trees on tolerant rootstocks may show mild to moderate symptoms that occasionally become severe if very high psylla populations occur in conjunction with other tree stresses.
Management:
i. There are many naturally occurring predators and parasites of pear psylla including green lacewings, brown lacewings, and minute pirate bugs.
ii. Adopting organically acceptable methods include biological and cultural control and sprays of approved oil, insecticidal soaps, azadirachtin, and kaolin clay.
Type # 3. Insect Transmission of Spiroplasma Diseases:
a. Corn Stunt:
Corn stunt is caused by the spiroplasma Spiroplasma kunkelli.
Symptoms:
Early symptoms consist of yellowish streaks in the youngest leaves. Later yellowing becomes more apparent. Infected plants remain stunted. This gives the plants a somewhat bunchy appearance at the top. Infected plants often have more ears, but the ears are smaller and bear little or no seed. Tassels of infected plants are usually sterile. There is also a proliferation of sucker shoots.
The corn stunt pathogen is the spiroplasma Spiroplasma kunkelli. It is transmitted in nature by the leafhoppers Dalbulus elimatus, D. maidis, and others. The leafhoppers must feed on diseased plants for several days before they can acquire the spiroplasma, and an incubation period of 2 to 3 weeks from the start of the feeding must elapse before the insects can infect healthy plants.
A feeding period of a few minutes to a few days may be required for the insects to inoculate the healthy plants with the spiroplasma. Plants show corn stunt symptoms 4 to 6 weeks after inoculation.
Management:
Corn stunt spiroplasma overwinters in Johnson grass and possibly other perennial plants. In the tropics, it perpetuates itself in continuous cropping’s of corn. The control of corn stunt depends on the planting of corn hybrids resistant to corn stunt.
b. Citrus Stubborn:
Citrus stubborn disease remains present in hot and dry areas. It affects leaves, fruits, and stems of all commercial varieties regardless of the rootstock. In general, affected trees show a bunchy, upright growth of twigs and branches, with short internodes and an excessive number of shoots.
Some of the affected twigs die back. The trees show slight to severe stunting. The leaves are small, often mottled or chlorotic. Excessive winter defoliation is common.
Affected trees bloom at all seasons, especially in the winter, but produce fewer fruits. Some of the fruit are very small and lopsided, frequently resembling acorns. Affected fruit tends to drop prematurely. Fruit are usually sour or bitter and have an unpleasant odor and flavor. Also, fruit from affected trees or parts of trees tend to have poorly developed and aborted seeds.
The pathogen is Spiroplasma citri. It is spread naturally in citrus orchards by several leafhoppers, such as Circulifer tenellus, Scaphytopius nitridus, and Neoaliturus haemoceps.
The control of citrus stubborn depends on the use of spiroplasma-free bud-wood and rootstocks, as well as early detection and removal of infected trees. Young citrus trees responded experimentally to treatment with tetracycline antibiotics, but this is not practiced commercially.