Plant Disease Diagnosis Lecture Notes PDF

PLANT DISEASE DIAGNOSIS (CP 102) LECTURE SUPPLEMENTAL NOTES Compiled by Rezel S. Borines, M.Sc. Joanne A. Lan...

PLANT DISEASE DIAGNOSIS (CP 102) LECTURE SUPPLEMENTAL NOTES Compiled by Rezel S. Borines, M.Sc. Joanne A. Langres, M.Sc. Rennielynn F. Canales, M.Sc. Elizabeth P. Parac, Ph.D. CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 1 CHAPTER I. INTRODUCTION The Plant Health A plant is healthy, or normal, when it can carry out its physiological functions to the best of its genetic potential. The meristematic (cambium) cells of a healthy plant divide and differentiate as needed, and different types of specialized cells absorb water and nutrients from the soil; translocate these to all plant parts; carry on photosynthesis, translocate, metabolize, or store the photosynthetic products; and produce seed or other reproductive organs for survival and multiplication. Plant health is a determining factor in crop yield and consequently in the income of the farmer. It is very important to manage the health of the crop so that profits are maximized. Healthy plants are vital to sustainable and profitable crop production and to the quality and cost of the nation’s supply of food, fuel, and fiber. Plant disease is only one of the factors that can affect the health of crop plants. Other factors include insect pests, weeds, nutrition, pesticides, soil conditions and the environment (Figure 1). All of these factors must be considered during the diagnostic process as each can affect the plant and cause symptoms similar to those caused by disease. Each factor can also potentially affect Figure 1. Key factors in maintaining the development of disease in the plant. Thus, diagnostic plant health plant pathologists should have an understanding of all of the factors that affect plant health and disease. 1.1 Weeds Many pests and pathogens persist on weed hosts when the susceptible crop host is absent. Therefore, effective weed control is an important control measure and a key part of integrated disease management (IDM). In addition, weeds growing with a crop will compete for water, nutrients and light, which will stress the crop and increase disease severity. 1.2 Insect Pest Feeding by invertebrate pests can cause damage to the plant similar to disease symptoms. For example, aphids, leaf hoppers, thrips, mites and whiteflies can cause damage CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 2 to the leaf similar to the symptoms of some foliar diseases. These pests also can act as vectors of viruses and bacteria. Stem borers and root grubs affect water uptake and can cause wilting that is similar to wilting caused by vascular wilt and root rot diseases. 1.3 Pesticides The application of pesticides can cause leaf damage, such as leaf burn and leaf spots. These symptoms can be confused with symptoms of leaf blight and leaf spots caused by many fungal and bacterial pathogens. Herbicides may stress plants, affecting their susceptibility to a pathogen. 1.4 Nutrition Poor nutrition commonly causes stunting and poor root growth (Figure 2.3). These symptoms are also caused by root rot pathogens. Other signs of mineral deficiencies and toxicities can also be similar to the symptoms of some diseases. For example, nitrogen deficiency causes leaf yellowing, particularly of the lower leaves. Leaf yellowing is also a symptom of root disease, which can also disrupt the uptake of nitrogen. Mineral deficiencies or toxicities can affect the susceptibility of plants to some pathogens. 1.5 Soil conditions Waterlogging (poor drainage), poor soil structure, hard clay soils and ‘plough pans’ (hard layers in the soil profile) can interfere with root growth. Stunting of the roots decreases the uptake of water and nutrients, causing stress on the whole plant. Stunting of the roots can also cause wilting and yellowing of the leaves, changes which are similar to the symptoms of many plant diseases. A plough pan can cause roots to grow laterally (turn sideways) reducing root function and growth; this stresses the plant, leading to favorable conditions for some pathogens. 1.6 Environment A variety of weather conditions can cause damage and stress to plants, and thus be detrimental to plant health. These include extremes of temperature, humidity and rain, as well as hail, flooding, drought and typhoons, lead to increased disease incidence and severity. High temperatures, low humidity and drought can cause severe wilting and plant death. Wet windy conditions facilitate infection and the spread of many fungal and bacterial leaf pathogens. Wet soil conditions favor Phytophthora and Pythium root rot diseases. Drought stress facilitates some root diseases, and stem and stalk rot problems. The combination of root rot disease and dry soil can kill plants. CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 3 1.7 Crop History An understanding of the history of the crop can help with the diagnosis of a disease. For example, the origin of the seed and whether it was treated with fungicide can provide an indication of whether a seed-borne pathogen may be affecting the crop. As discussed above it is important to understand the history of weather conditions prior to a disease outbreak. Cool wet conditions favor many root rot pathogens but the plant may tolerate some damage to the roots under these conditions as transpiration rates are low. However, if the weather turns hot and transpiration rates are high, the diseased plant can quickly wilt and die. An earlier infestation of a virus vector in a crop could indicate that a virus carried by the vector has infected the crop and is responsible for the symptoms observed. Knowledge of the previous crops and their diseases can also provide a guide to potential diseases in the current crop. For example, some rotations will increase the severity of particular diseases caused by soil-borne pathogens. For example, successive crops in the family Solanaceae are likely to increase bacterial wilt caused by Ralstonia solanacearum. What is a disease? At the end of this lesson, the student may able to learn the concept of plant disease, understand the disease triangle and factors that cause plant diseases. The term plant disease refers to any disturbances brought about by a pathogen, or an environmental factor which interferes with manufacture, translocation, or utilization of food, mineral nutrients and water in such a way that the affected plant changes in appearance and yields less than a normal healthy plant of the same variety. Some plant diseases are easily recognizable based on specific symptoms expressed on host plants but others may not, hence a structured diagnostic process is recommended. Plant diseases are either caused by abiotic and biotic agents as follows: Other definitions: Any physiological malfunctioning caused by animate agents (Whetzel, 1929) A dynamic interaction between an organism and its environment which results in abnormal physiological and often morphological & neurological changes in organism (Merill, 1980) The senses of invisible and visible responses of plant cells and tissues to a pathogenic microorganism or environmental factor that result in adverse changes CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 4  in the form, function or integrity of the plant and may lead to partial impairment or death of the plant or its parts (Agrios, 1997) Abiotic factors that cause plant diseases Abiotic disorders are usually caused by temperature, moisture, soil pH, air quality, light regime and nutrition. If any of these factors fluctuate from the optimum range for a given plant species, plant growth might be adversely affected. Noninfectious diseases occur in the absence of pathogens and cannot, therefore, be transmitted from diseased to healthy plants. Noninfectious diseases may affect plants in all stages (e.g., seed, seedling, mature plant, or fruit), and they may cause damage in the field, in storage, or at the market. The symptoms caused by noninfectious diseases vary in kind and severity with the particular environmental factor involved and with the degree of deviation of this factor from its normal. Symptoms may range from slight to severe, and affected plants may even die. The diagnosis of noninfectious diseases is sometimes made easy by the presence of characteristic symptoms known to be caused by the lack or excess of a particular factor on the plant The following are the causes of Abiotic Plant Diseases Excessively low temperature Too high temperature Lack of oxygen Too much or too little light Adverse meteorological conditions - strong winds, heavy rains, lightning Excess or deficiency of nutrients Unfavorable pH Improper use of pesticides Improper agricultural practices Lack or excess of soil moisture Naturally occurring toxic chemicals Biological Agents of Plant diseases The following are the biological agents of plant disease Bacteria and mollicutes (prokaryotes, unicellular) Fungi and fungal-like protists (multi-cellular) Viruses and viroids (subcellular entities) Nematodes (worm-like microorganism) CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 5  Importance of Plant Disease/ Historical Perspective Like human beings and other animals, plants are subject to diseases. Plant diseases are of paramount importance to humans because they damage plants and plant products on which humans depend for food, clothing, furniture, the environment, and, in many cases, housing. In order to maintain a sufficient food supply for the world's population, it is necessary for those involved in plant growth and management to find ways to combat plant diseases that are capable of destroying crops on a large scale. Plant Diseases with Global Economic Impact a. Potato Late Blight (Phytophthora infestans) This disease caused a Great Famine, also called Irish Potato Famine, Great Irish Famine, or Famine of 1845–1849, famine that occurred in Ireland in 1845–1849 when the potato crop failed in successive years. The crop failures were caused by late blight, a disease that destroys both the leaves and the edible roots, or tubers, of the potato plant. The causative agent of late blight is the water mold Phytophthora infestans. The Irish famine was the worst to occur in Europe in the 19th century. It was reported that in 1840 the population of Ireland was 8 million which was reduced to 4 million after the famine. b. Rice brown spot (Helminthosporium oryzae) Brown spot has been associated with two major epidemics in India, the first in 1918–19, in the Krishna Godavari delta and the second, during 1942 in today’s India and Bangladesh. The latter of which has been associated with the Great Bengal Famine. Death by starvation of an estimated two million people living in Bengal in India and Bangladesh were recorded. In the last year of Second World War (1943) Bengal had to face a serious famine. One of the reasons to which this famine has been attributed was the loss in yield of the rice crop due to attack of Helminthosporium leaf spot which had been affecting the crop for the last several years. c. Coffee leaf rust (Hemeileia vastatrix) Coffee in Sri Lanka became uneconomic -last part of the 19th century due to the disease. The farmers turned to growing tea. It affected the entire farms and bring to 80% drop. Sri Lanka was one of the world's largest coffee producers, exporting over 100 million pounds annually. CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 6  d. Lethal Yellowing of Coconut Palms (Candidatus Phytoplasma) The lethal yellowing disease was first found in mainland Florida in 1971 and killed 15,000 coconut palm trees by1973, 40,000 by 1974, and, by 1975, 75% of the coconut palm trees in Dade County were dead or dying from the disease. Tremendous losses of palm trees occurred in many other countries. In the Philippines alone, enormous economic losses have been brought by plant disease. a. Cadang-cadang Disease of Cononut (Viroids) The disease has cost the country a loss of over two hundred milion dollars since it was observed in 1918. b. Downy mildew of corn (Peronosclerospora philipinensis) This disease is known to be the nemesis of corn, used to destroy as much as 95% of the corn crop before its control by chemical seed treatment was discovered in 1978. The loss amounted to over 170 million pesos annually. c. Rice tungro (Rice tungro bacilliform virus and/ Rice tungro spherical virus) The tungro disease of rice affected 70,000 hectares of rice fields in 1971 alone, causing a loss of 1.22 million cavans of rough rice valued at P30,357,000. d. Bunchy top Disease in Abaca (Abaca bunchy top virus/ Banana bunchy top virus) An estimated loss in fiber yield in the Bicol region [Philippines] and equivalent values in pesos as a result of the widespread occurrence of bunchy-top and mosaic were enormous. On a region-wide mean disease incidence of 5.19 percent, the estimated yield loss in fiber yield was 833,587.99 kg valued at Php18,338,935.78. The province of Sorsogon appeared to be the hardest hit with an equivalent loss of Php9,458,893.40. CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 7  Terms and Definition Pathogen Any agent that cause disease especially the biotic or living agent. Parasite An organism which depends wholly or partly on another living organism for its food. Parasitism The removal of food by a parasite from its host. Saprophyte An organism that uses dead organic material for food. Obligate parasite An organism that in nature can grow and multiply only in living organisms also called as biotrophs. Facultative parasite A parasite that live most of the time and thrive on dead organic matter, but under certain circumstances, may attack living plants and become parasitic. Also called necrotrophs. Facultative saprophyte A parasites that live most of the time or most of their life cycles as parasites, but under certain conditions, may grow saprophytically on dead organic matter. Also called semibiotrophs. Host A plant that is being attacked by a parasite and from which the parasite obtains its nutrients. Suscept Any plant that can be attacked by a pathogen; a host plant. Pathogenicity The capability of a pathogen to cause disease. Virulence Quantitative amount of disease that an isolate (the pathogen) can cause in a group of plants. Disease cycle The chains of events involved in disease development including the stages of development of the pathogen and the effect of the disease on the host. Inoculation The arrival or transfer of a pathogen onto a host. Inoculum The pathogen or its parts that can cause infection; that portion of individual pathogens that are brought into contact with the host. Infection The establishment of a parasite within a host plant. Infested Containing great numbers of insects, mites, nematodes, etc. as applied to an area or field. Disinfectant A physical or chemical agent that frees a plant; organ; or tissue from infection. Symptoms The external and internal reactions or alterations of a plant as a result of disease. Symptom complex Different symptoms exhibited by the plant. Signs The pathogen or its parts or products seen on a host plant. CSU-CAA Supplemental Lecture Notes | CP 102, Rev. 0, 06-01-20 8