CHAPTER TEN:
PLANT DISEASES
Muhammed, Habiba Maikudi
Disease in plants can be defined as any disturbance brought about by a pathogen or due to environmental factors which interferes with the physiological function of a plant. Agrios (2005) has defined disease in plants as a series of invisible and visible response of plant and tissues to a pathogenic microorganism or environmental factors that result in adverse change in the form, function, or integrity of the plant parts. According to Nutter et al., (2006), disease is a malfunctioning process that is caused by continuous irritation.
10.1 Concept of Disease in Plants
A plant is healthy when it can carry out its physiological functions to the best of its genetic potential. These functions include:
1. Normal cell division
2. Differentiation and development
3. Absorption of water and minerals from the soil.
4. Photosynthesis and translocation of photosynthetic products.
5. Reproduction and the storage of food supplies.
Whenever plants are disturbed by pathogens or by environmental factors one or more of these functions are altered with beyond certain deviation from the normal, then the plants become diseased. The specific mechanisms by which diseases are produced vary considerably with the causal agent and sometimes with the plant. At first the reaction of the plant to the disease-causing agent is at the site of affliction, which is of chemical nature and invisible. Soon, however, the reaction becomes more widespread and histological changes take place that manifest themselves macroscopically and constitute the symptoms of the disease.
10.2 Classification of Plant Diseases
Plant diseases are classified according to the following criteria:
1. Symptoms they cause e.g. Root rots, cankers, wilts, leafspots, blights, mosaic etc.
2. Plant organ they affect e.g. root diseases, stem diseases, fruit diseases etc.
3. Types of plants affected e.g. field crops diseases, vegetable diseases, fruit tree diseases, diseases of ornamental plants etc. (Agrios, 2005).
However, the most useful criteria for classifying plant diseases are the type of pathogen that causes the diseases. On this basis plant diseases are classified as:
1. Infectious plant diseases which include:
i. Diseases caused by fungi
ii. Diseases caused by bacteria
iii. Diseases caused by mycoplasmas
iv. Diseases caused by parasitic higher plants
v. Diseases caused by viruses
vi. Diseases caused by nematodes
vii. Diseases caused by protozoa.
2. Non infectious diseases
i. Inadequate temperature
ii. Lack or excess light
iii. Inadequate oxygen
iv. Nutrient deficiencies
v. Mineral toxicities
vi. improper agricultural practices etc.
Indirectly, environmental factors that cause a plant to be stressed may result in the plant's gradual decline. Decline results in the plant being more susceptible to disease organisms. Because of this, diagnosing plant diseases can be tricky. The real cause of a problem may be the stress factors, with the disease simply being a secondary factor. Three critical factors or conditions must exist for disease to occur: a susceptible host plant, a pathogen, and the right mix of environmental conditions. The relationship of these factors is called the disease triangle. If only a part of the triangle exists, disease will not occur. Understanding the disease triangle helps us understand why most plants are not affected by the many thousands of diseases that exist. Pathogens are microorganisms that cause disease. Because they are living, they are called biotic (bye-AH-tick) agents or causes. Pathogens can be fungi (FUN-geye), bacteria, viruses, mycoplasmas (MY-crow-plas-mahs) or nematodes (KNEE-ma-toads). Each has a different life cycle, which includes an infectious stage (Agrios, 2005).
Most pathogens are host-specific to a particular plant species, genus or family. For instance, blackspot of rose will not attack marigolds or lettuce. Some diseases, such as the powdery mildews, produce similar symptoms on different plants. However, the fungi involved are usually host-specific. The rose powdery mildew fungus will not infect zinnias or turfgrass or vice-versa.
A susceptible host has a genetic makeup that permits the development of a particular disease. The genetic defense against a disease is called disease resistance. This resistance can be physical characteristics of the plant (fuzzy or waxy leaf surfaces), chemical characteristics (enzymes that kill pathogens and lack of enzymes) and growth patterns (ability to block off diseased tissue or outgrow damage). Plants also may be disease-tolerant. Even though infected with a disease, they can grow and produce a good crop or maintain an acceptable appearance. The plant outgrows the disease and symptoms are not apparent or at a damaging level (Zadoks, 2001).
Certain environmental conditions must exist for disease pathogens to cause infection. The specific conditions vary for different pathogens. High moisture and specific temperature ranges, for example, are necessary for many fungal diseases. These conditions must continue for a critical period of time while the pathogen is in contact with the host for infection to occur. Moisture, temperature, wind, sunlight, nutrition and soil quality affect plant growth. If one of these factors is out of balance for the culture of a specific plant, that plant may have a greater tendency to become diseased. For example, lilacs growing in shade are more likely to be infected with powdery mildew than those growing in full sunlight. Often gardeners believe that their plants have become diseased overnight. This may be true in the case of damping-off. More often, however, much has occurred before symptoms are seen. There are five stages in disease development: inoculation, incubation, penetration, infection and symptoms (Zadoks,2001).
INOCULATION
The pathogen must be introduced (inoculated) to the host plant. Most pathogens cannot move on their own, but must be carried to the host plant. This is done by rain, wind, insects, birds and people (Nutter et al., 2006).
INCUBATION
The second stage of disease development is incubation. The pathogen changes or grows into a form that can enter the new host plant. In many fungal diseases, the pathogen arrives on the plant as a spore which must germinate before it can grow into the plant.
PENETRATION
The third stage is penetration or the point at which the pathogen actually enters the host plant. Once the fungal spore germinates, it sends out thread-like tubes call hyphae. These penetrate the plant through wounds or natural pores. Wounding roots of bedding plants during transplanting provides entry for root-rotting fungi. The mouthparts of an insect also result in openings for penetration.
Figure 1.Penetration of disease in plant.
Source: Agrios, 2005
Indirect Penetration:
Penetration of plant tissues by a pathogen through natural openings
INFECTION
The fourth stage is infection. The pathogen grows within the plant and begins damaging the plant tissue.As the pathogen consumes nutrients, the plant reacts by showing symptoms. Symptoms are evidence of the pathogens causing damage to the plant. Symptoms include mottling, dwarfing, distortion, discoloration, wilting, and shriveling of any plant part (Nutter et al., 2006).
SYMPTOMS
As the pathogen consumes nutrients, the plant reacts by showing symptoms. Symptoms are evidence of the pathogens causing damage to the plant. Symptoms include mottling, dwarfing, distortion, discoloration, wilting, and shriveling of any plant part. As shown in figure 2.
Figure 2. Symptoms of a diseased plant.
Source: Agrios, 2005
PLANT PATHOGENS
FUNGI
The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes. The fungi reproduce both sexually and asexually via the production of spores and other structures. Spores may be spread long distances by air or water, or they may be soil borne. Many soil inhabiting fungi are capable of living saprotrophically, carrying out the part of their lifecycle in the soil. These are known as facultative saprotrophs (Wright, 2012).
Fungal diseases may be controlled through the use of fungicides and other agriculture practices, however new races of fungi often evolve that are resistant to various fungicides.
Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells (Wright, 2012).
Figure 3: Rice blast, a necrotrophic fungus
Source: Agrios, 2005
Significant fungal plant pathogens include:
Ascomycetes
· Fusarium spp. (causal agents of Fusarium wilt disease) e.g .Fusarium oxysporum in guava and tomato fruits.
Thielaviopsis spp. (causal agents of: canker rot, black root rot, Thielaviopsis root rot)
Verticillium spp.
Magnaporthegrisea (causal agent of blast of rice and gray leaf spot in turf grasses) (Camire et al., 2009).
Basidiomycetes
· Rhizoctonia spp.( causal agent of rice sheath blast, and jute plant).
Phakosporapachyrhizi (causal agent of soybean rust)
Puccinia spp. (causal agents of severe rusts of virtually all cereal grains and cultivated grasses)
Oomycetes
Theoomycetes are not true fungi but are fungus-like organisms They include some of the most destructive plant pathogens including the genus Phytophthora, which includes the causal agents of potato late blight and sudden oak death. Particular species of oomycetes are responsible forroot rot. (Camire et al., 2009).
Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies. Oomycetes are capable of using effect or proteins to turn off a plant's defenses in its infection process. Plant pathologists commonly group them with fungal pathogens (Camire et al., 2009)..
Significant oomycete plant pathogens
· Pythium spp.(causal agent of ginger rhizome rot and soft rot in tomato).
Phytophthora spp.; including the causal agent of the Great Irish Famine (1845–1849)
Bacteria
Figure 4. Crown gall disease caused by an Agrobacterium
Source: Agrios, 2005
Most bacteria that are associated with plants are actually saprotrophic, and do no harm to the plant itself. However, a small number, around 100 known species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world. Most plant pathogenic bacteria are rod-shaped (bacilli). In order to be able to colonize the plant they have specific pathogencity factors. Five main types of bacterial pathogenicity factors are known: uses of Cell wall-degrading enzymes, Toxins, Effect or proteins, Phytohormones and Exopolysaccharides ( Jackson, 2009).
Pathogens such as Erwinia, use Cell wall-degrading enzymes to cause soft rot. Agrobacterium changes the level of auxins to cause tumours with phytohormones. Exopolysaccharides are produced by bacteria and block xylem vessels, often leading to the death of the plant.
Significant bacterial plant pathogens:
o Xanthomonas spp (causal agent of bacterial blight in rice).
Pseudomonas spp. (causal agent of bacterial wilt in potato).
Pseudomonas syringae in tomato, causes tomato plants to produce less fruit, and it "continues to adapt to the tomato by minimizing its recognition by the tomato immune system (Jackson, 2009).
Phytoplasmas (Mycoplasma-like organisms) and spiroplasmas
Phytoplasma and Spiroplasma are a genre of bacteria that lack cell walls, and are related to the mycoplasmas, which are human pathogens. Together they are referred to as the mollicutes. They also tend to have smaller genomes than true bacteria. They are normally transmitted by sap-sucking insects, being transferred into the plants phloem where it reproduces.
Figure 5.Tobacco mosaic virus
Source: Agrios, 2005
Viruses, viroids and virus-like organisms
There are many types of plant virus, and some are even asymptomatic. Under normal circumstances, plant viruses cause only a loss of crop yield. Therefore, it is not economically viable to try to control them, the exception being when they infect perennial species, such as fruit trees (Popp and Hantos, 2011).
Most plant viruses have small, single-stranded RNA genomes. These genomes may encode only three or four proteins: a replicas’, a coat protein, a movement protein, in order to allow cell to cell movement though plasmodesmata, and sometimes a protein that allows transmission by a vector. Plant viruses must be transmitted from plant to plant by a vector. This is often by an insect (for example, aphids), but some fungi, nematodes, and protozoa have been shown to be viral vectors.eg. Rice turgo, maized warf mosaic, downy mildew of lettuce,citrus tristeza and bean mosaic (Popp and Hantos, 2011).
Nematodes
Figure 6.Root-knot nematode galls
Source: Agrios, 2005
Nematodes are small, multicellular wormlike creatures. Many live freely in the soil, but there are some species that parasitize plant roots. They are a problem in tropical and subtropical regions of the world, where they may infect crops. Potato cyst nematodes (Globoderapallida and G. rostochiensis). Root knot nematodes have quite a large host range, whereas cyst nematodes tend to be able to infect only a few species. Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle.e.g. Root knot of vegetables, cereal cyst nematodes, soybean cyst nematode and white tip disease of rice (Sharma and Thakur, 2007).
Protozoa
There are a few examples of plant diseases caused by protozoa. They are transmitted as zoospores that are very durable, and may be able to survive in a resting state in the soil for many years. They have also been shown to transmit plant viruses. When the motile zoospores come into contact with a root hair they produce a plasmodium and invade the roots.
Parasitic higher plants
Parasitic plants such as mistletoe and dodder are included in the study of phytopathology. Dodder, for example, is used as a conduit either for the transmission of viruses or virus-like agents from a host plant to a plant that is not typically a host or for an agent that is not graft-transmissible.eg. causal agents witch weed disease of sugar cane, rice, and small grains , broomrapes disease of tomato and tobacco, dodder disease in onions, sugar beets and potato (Sharma and Thakur, 2007),
10.3 Diagnosis of Plant Diseases
For diagnosis of a plant disease, it is important to first determine whether the disease is caused by a pathogen or due to environmental factors. For disease caused by pathogens e.g. Fungi, bacteria, parasitic higher plants, nematodes, viruses are characterized by the presence of these pathogens on the surface of these plants or inside the plants. The presence of such pathogens at an active state on the surface of the plant would indicate that they are probably the cause of the disease. However, if no pathogen could be found transmitted from a diseased plant, then it would have to be assumed that the diseases is caused by environmental factor (Zadoks, 2001).
10.3.1 Identification of a Previously Unknown Disease
When a pathogen is found on a diseased plant, the pathogen is identified by reference to special manuals, but, however, if the pathogen is known to cause such a disease, then the diagnosis of the disease may be considered completed. If, however, the pathogen found to be the cause of the disease but no previous reports have been made, then the following steps are taken.
1. The pathogen must be found to be associated with the disease in all the diseased plants.
2. The pathogen must be isolated and grown in pure culture on nutrient media.
3. The pathogen from the pure culture must be inoculated on healthy plants of the same species, and it must produce the same disease on the inoculated plants.
4. The pathogen must be isolated in pure culture again and its characteristics must be exactly like those observed.
The above steps are known as Koch’s postulates, if all the above steps have been followed then the isolated pathogen is identified as the pathogen responsible for the disease (Agrios, 2005).
References
Agrios, G.N. (2005), Plant pathology, Fourth Edition, Academic Presss, San Diego; pp.635.
Camire, M. E., Kubow, S., and Donnelly, D. J. (2009). Potatoes and human health. Critical reviews in food science and nutrition, 49(10), 823-840.
Jackson, R. W. (2009). Plant pathogenic bacteria: genomics and molecular biology. Caister Academic Press.
Nutter, F. W., Esker, P. D., and Netto, R. A. C. (2006). Disease assessment concepts and the advancements made in improving the accuracy and precision of plant disease data. European Journal of Plant Pathology, 115, 95-103.
Popp, J., and Hantos, K. (2011). The impact of crop protection on agricultural production. Studies in Agricultural Economics, 113(1), 47-66.
Sharma, S., and Thakur, M. (2007). Role of plant quarantine in the management of pest organisms–A review. Agricultural Reviews, 28(4), 235-244.
Wright, B. D. (2012). Grand missions of agricultural innovation. Research Policy, 41(10), 1716-1728.
Zadoks, J. C. (2001). Plant disease epidemiology in the twentieth century: A picture by means of selected controversies. Plant Disease, 85(8), 808-816.