Transmission of Plant Diseases by Insects
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TRANSMISSION OF PLANT DISEASES BY INSECTS George N. Agrios University of Florida Gainesville, Florida, USA Plant diseases appear as pathogenic organism (some fungi, some necrotic areas, usually spots of various bacteria, some nematodes, all protozoa shapes and sizes on leaves, shoots, causing disease in plants, and many and fruit; as cankers on stems; as viruses) depends on for transmission blights, wilts, and necrosis of shoots, from one plant to another, and on which branches and entire plants; as some pathogens depend on for survival discolorations, malformations, galls, and (Fig. 1). root rots, etc. Regardless of their The importance of insect appearance, plant diseases interfere transmission of plant diseases has with one or more of the physiological generally been overlooked and greatly functions of the plant (absorption and underestimated. Many plant diseases in translocation of water and nutrients from the field or in harvested plant produce the soil, photosynthesis, etc.), and become much more serious and thereby reduce the ability of the plant to damaging in the presence of specific or grow and produce the product for which non-specific insect vectors that spread it is cultivated. Plant diseases are the pathogen to new hosts. Many generally caused by microscopic insects facilitate the entry of a pathogen organisms such as fungi, bacteria, into its host through the wounds the nematodes, protozoa, and parasitic insects make on aboveground or green algae, that penetrate, infect, and belowground plant organs. In some feed off one or more types of host cases, insects help the survival of the plants; submicroscopic organisms such pathogen by allowing it to overseason in as viruses and viroids that enter, infect, the body of the insect. Finally, in many spread systemically and affect the cases, insects make possible the growth of their host plants; parasitic existence of a plant disease by higher plants which range from about an obtaining, carrying, and delivering into inch to several feet in size and penetrate host plants pathogens that, in the and feed off their host plants. Plant absence of the insect, would have been diseases are also caused by abiotic, unable to spread, and thereby unable to environmental factors such as nutrient cause disease. It is offered as a guess deficiencies, extremes in temperature that 30-40% of the damage and losses and soil moisture, etc. that affect the caused by plant diseases is due to the normal growth and survival of affected direct or indirect effects of transmission plants. and facilitation of pathogens by insects. Of the aforementioned causes of Insects and related organisms, disease, many of the microscopic such as mites, are frequently involved in organisms and of the viruses are the transmission of plant pathogens transmitted by insects either accidentally from one plant organ, or one plant, to (several fungi and bacteria) or by a another on which then the pathogens specific insect vector on which the cause disease. Equally important is that insects can and do transmit pathogens pathogen with the plant sap they eat. among plants from one field to another, Subsequently, the pathogen circulates in many cases even when the fields are through the body of the insect until, with several to many miles apart. Almost all or without further multiplication in the types of pathogens, that is, fungi, insect, the pathogen reaches the bacteria, viruses, nematodes, and salivary glands and the mouthparts of protozoa, can be transmitted by insects. the insect through which it is injected Insects transmit pathogens, such as into the next plant on which the insect many fungi and bacteria, mostly feeds (Fig. 3). externally on their legs, mouthparts, and bodies. Almost all plant pathogenic Role of insects in bacterial diseases viruses, all phytoplasmas, xylem- and of plants phloem-inhabiting fungi and bacteria, In most plant diseases caused by some protozoa, and some nematodes plant pathogenic bacteria (especially in are also transmitted by insects, and they those that cause spots, cankers, blights, are usually carried by the insect galls, or soft rots, bacteria), which are internally. The insects that transmit fungi produced within or between plant cells, and bacteria externally on their bodies escape to the surface of their host and legs belong to many orders of plants as droplets or masses of sticky insects. On the contrary, the insects that exudates (ooze). The bacteria exudates transmit the other pathogens listed are released through cracks or wounds above internally are very specialized in the infected area, or through natural and specific for the pathogen they openings such as stomata, transmit and belong to a certain species nectarthodes, hydathodes, and or genus of insects (Fig. 2). sometimes through lenticells, present in Insects transmit pathogens in the infected area. Such bacteria are three main ways. 1) Many insects then likely to stick on the legs and transmit bacteria and fungal spores bodies of all sorts of insects, such as passively by feeding in or walking flies, aphids, ants, beetles, whiteflies, through an infected plant area that has etc., that land on the plant and come in on its surface plant pathogenic bacteria contact with the bacterial exudates. or fungal spores as a result of the Many of these insects are actually infection. The bacteria and spores are attracted by the sugars contained in the often sticky, cling to the insect as it bacterial exudate and feed on it, thereby moves about, and are carried by it to further smearing their body and other plants or parts of the same plant mouthparts with the bacteria-containing where they may start a new infection. 2) exudate. When such bacteria-smeared Some insects transmit certain bacteria, insects move to other parts of the plant fungi, and viruses by feeding on infected or to other susceptible host plants, they plant tissues and carrying the pathogen carry on their body numerous bacteria. If on their mouthparts as they visit and the insects happen to land on a fresh feed on other plants or plant parts. 3) wound or on an open natural opening, Several insects transmit specific viruses, and there is enough moisture on the phytoplasmas, protozoa, nematodes, plant surface, the bacteria may multiply, and xylem- and phloem-inhabiting move into the plant, and begin a new bacteria by ingesting (sucking) the infection. The same happens if the insects happen to create a fresh wound Bacterial soft rots on the plant. Bacterial soft rots cause The type of insect transmission of tremendous losses worldwide, bacteria is probably quite common and particularly in the warmer climates and widespread among bacterial diseases of the tropics. They are caused primarily plants, but it is passive and haphazard, by the bacterium Erwinia carotovora pv. depending a great deal on the carotovora, to some extent by availability of wounds or moisture on the Pseudomonas fluorescens and Ps. plant surface. In any case, there are few chrysanthemi, and, occasionally, by data on how frequently such species of Bacillus and Clostridium. The transmission occurs, and many last two genera of bacteria cause rotting conclusions about it are the result of of potatoes and of cut fleshy leaves in conjecture. A further point that has been storage while Pseudomonas fluorescens made is that insects which, whether and Ps. chrysanthemi cause soft rots of above or below ground, wound the host many fleshy fruits and fleshy plant organs (roots, shoots, fruit, etc.) by vegetables. The species E. c. pv. feeding or by ovipositing in them, carotovora causes the vast majority of increase the probability of transmission soft rots on fleshy plant organs of any of plant pathogenic bacteria. This occurs type (leave, blossoms, fruit, stems, or because such insects place the roots), especially in storage and under bacteria, with their mouthparts or the cover or in plastic bags. Affected fleshy ovipositor, in or around wounded plant fruits are, for example, strawberries and cells, where they are surrounded by a other berries, cantaloupes, peaches, suspension of nutrients (plant cell sap) pears, etc.; vegetables, for example, in the absence of active host defenses tomatoes, potatoes, spinach, celery, and where they can multiply rapidly and onions, cabbage, etc.; and ornamentals, subsequently infect adjacent healthy for example, cyclamen, iris, lily, etc. tissues. Nearly all fleshy vegetables are subject Numerous plant diseases could to bacterial soft rots. The soft rot be listed among those in which bacteria bacteria enter the plant organ through a are spread by insects passively as wound, sometimes in the field but more described above, for example, the commonly during storage, and there bacterial bean blights, fire blight of apple they multiply rapidly, secrete enzymes and pear, citrus canker, cotton boll rot, that separate the cells from each other crown gal, bacterial spot and canker of and macerate the plant cell walls, which stone fruits, etc. In several bacterial causes the tissues to become soft and diseases, however, the causal to rot. In many cases, these bacteria are bacterium has developed a special accompanied in the rotting tissues by symbiotic relationship with one or a few other saprophytic bacteria that further specific types of insects and depends a degrade the softened plant tissue and great deal on these insects for its cause it to give off a foul odor. In all spread from infected to healthy host cases, rotting tissues become soft and plants. Some of the better known watery, and slimy masses of bacteria bacterium - insect associations are ooze out from cracks in the tissues. described briefly below. The soft rotting bacteria survive in infected fleshy organs in storage and in the field, in plant debris, in infected Otitidae), the seedcorn maggot, and the roots and other plant parts of their hosts, onion bulb fly, Eumerus strigatus in ponds and streams from where (Fallen) (Diptera: Syrphidae) and the irrigation water is obtained, and to some soft rot of onion; and the iris borer, extent in the soil and in the pupae of Macronoctua onusta (Grote) several insects.