Monthly Archives: December 2015

BASIC SCIENCE OF PLANT DISEASES

INTRODUCTION
A disease in plants can be defined as any disturbance brought about by an agency/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 or yields less than a normal, healthy plant of the same variety.
CAUSES OF PLANT DISEASES
Plant diseases are of two types viz., infectious and non-infectious. The infectious type are caused by fungi, bacteria and viruses. Nutritional deficiencies, mineral toxicities, lack or excess of soil moisture and light, low or too high temperatures, soil acidity or alkalinity constitute the non-infectious type.

Usually, a disease causes a progressive and continuous disturbance of cellular activities that eventually become manifest as symptoms. A symptom is a visible or otherwise detectable abnormality arising from disease. Disease symptoms should be closely observed, as they often give indications about the cause of the disease and thus have a diagnostic value. Environmental factors must always be taken into account. Symptoms may either be mild in terms of extension or intensity, or they may be acute or severe, the latter usually leading to the death of plants or plant parts. The spreading of a disease depends on two factors, i.e. time and environment. The time determines the course of the disease as the pathogen population increases with the passage of time. Environmental factors, such as climatic and soil conditions, and cultivation methods, have a great influence on the expression of the disease’s symptoms.
(1) NON-INFECTIOUS DISEASES
Many serious disorders are caused by external physical factors which adversely affect the proper growth of plants. Very often, the disorder is a direct result of a deficient or excessive supply of an essential growth factor, such as light, oxygen, soil moisture and heat. These are indispensable for the various life processes in plants including water and nutrient uptake, respiration and photosynthesis. A strong interaction exists between these growth factors. Generally, each plant has the capacity to balance its requirement for each of them. However, abrupt changes in environmental conditions or a severe deficiency or injurious overabundance of one or more of the growth factors may disrupt a plant’s life process and bring about disease symptoms. Since these disorders are not caused by parasites, they are not infectious.

Light: The requirement for light is very variable, some plant species prefer shade, others full sunlight. In many species the total hours of daylight – long day versus short day varieties determine the transgression from the vegetative to the generative phase. However, too much light, especially in combination with high temperature and lack of water, produces wilting or scalding in broad-leaved plants. Too little light produces chlorosis and etiolated or “lanky” growth.

Oxygen: An adequate supply of oxygen to above – ground parts of the plants is essential for effective respiration. A shortage of oxygen in the soil owing to poor soil structure, compaction or water-logging hampers the development of a healthy root system.

Water: Regular availability of water is essential for photosynthesis and metabolic processes in the plants. Plants absorb water mainly through the root hairs, this assists in the uptake of nutrients from the soil and their distribution throughout the plant. Evaporation of water from the leaves aids in cooling of leaf surfaces. Water contained within the walls of plant cells lends firmness (turgor) to plants, juicy fruits and succulent leaves may contain more than 90% water. A low relative humidity (dry air) may accelerate evaporation of water from the leaves and cause wilting when water pressure (turgor) cannot be maintained in the plant by replenishment from the roots. Early stages of wilting may be irreversible and the tissues may be damaged permanently.

Temperature: The supply of heat, or the lack of it, determines to a large extent the functioning of life processes in plants. Plants have an optimum temperature range for healthy development and a particular total requirement for heat in order to reach their full productivity.

Nutrients: Overabundance and deficiency of nutrients may cause a whole range of symptoms from discoloration to rot, die-back or stunted growth. Commonly, the productivity of a plant is lowered and its resistance to parasites weakened. The appearance of characteristic deficiency symptoms on leaves does not necessarily mean that the nutrient is in short supply in the soil. In fact, it might be sufficiently available, but its uptake may be impaired by an unfavourable acidity rate (pH) or by the shortage of another element. Soil salinity or alkalinity are serious problems threatening irrigated agriculture, where no possibilities exist to flush and drain the salt from the top layer of the soil. Continued fertilizing with ammonium sulphate may lead to high acidity of the soil and in time to toxic accumulation of elements such as iron, manganese and aluminium.

SYMPTOMS OF PLANT DISEASES
The way plant diseases express themselves by way of specific symptoms often holds important clues for diagnosis and treatment. The diagnosis must, however, take into account all environmental conditions, since these affect symptom expression. For instance, soil-borne diseases and nematodes may inhibit root development and thus cause poor growth. Likewise, a hard and compact soil structure near the surface may prevent the development of plant roots with a similar result. The following are the symptoms that generally occur with plant diseases:

Damping off– Sudden collapse and death of seedlings in the seed bed or field, e.g. damping off in tomato, cabbage and cotton.

Wilt– Sudden or gradual wilting and death of grown up plants as a result of disturbance in the vascular systems e.g. cotton wilt, green beans wilt, fusarium/bacterial wilt in peas.

Spot– Localised necrotic lesions on leaves/fruits consisting of dead or collapsed tissues, e.g. Brown spot of rice.

Blight– General and extremely rapid browning of leaves, branches, twigs and floral parts resulting in scorching and death of affected part.

Mildew– White, grey, brownish patches of varying size on leaves, stem and fruit usually covered with mycelium and fructification of the fungus e.g. Downy mildew of rose cut flowers and powdery mildew of cucurbits.

Canker– sunken out growths on leaves, stem, fruit which may be either smooth or rough e.g. citrus canker.

Scab– Roughed or crust like lesion giving a freckled appearance of the diseased organ e.g. potato scab.

Galls– Malformations of globose, elongated or irregular shape as a result fo excessive cell division and cell enlargement e.g. crown galls on rose cut flowers.

Mosaic– Alteration of light green, yellow, or white patches with the normal green of the leaves or fruits e.g. tomato mosaic.

Yellows– Uniform discolouration or chlorosis of the foliage without any sporting pattern.

Leaf curl– Distortion, thickening and curling of leaves e.g. tomato leaf curl.

Ring spot– Appearance of chlorotic or necrotic rings on the leaves and fruit e.g. tomato sported wilt virus.

Vein banding– Retention of bands of green tissue along the veins while the tissue between the veins is chlorotic e.g. vein banding mosaic.

Vein clearing– Destruction of chlorophyll adjacent or in the veinal tissue e.g. yellow vein mosaic

Witches broom– Profuse upward branching of twigs e.g. potato witches broom.

(2) INFECTIOUS DISEASES
1. PLANT PATHOGEN FUNGI
Fungi are nucleated, spore bearing, achlorophyllous organisms which generally reproduce sexually and whose usually filamentous, branched somatic structures are typically surrounded by cell walls containing cellulose or chitin or both. Fungi reproduce by means of spores which are specialised propagated or reproductive bodies which are formed asexually or sexually. Asexual reproduction is more important for the propagation of the species, as it results in production of numerous individuals and is usually repeated several times during the season. The spread of the disease mainly takes place with the help of asexually formed spores like zoospores, conidia and oidia. Sexual reproduction normally results in the production of resting spores like oospores, ascospores etc, which help the pathogen in tiding over unfavourable conditions. sexual stage of many fungi is produced once in a crop season. In general, it is the imperfect state that is the active pathogen, the perfect state (sexual) occurring in the terminal stages of the fungus spore and certain amount of growth which in turn depend moisture, temperature etc, which vary for different pathogens. Some fungi penetrate directly through cuticle and epidermis, others through stomata and wounds. The success of a fungus as a pathogen also depends on efficient transmission. Seed or other vegetative propagules, soil debris of a previous crop, weed hosts, alternate hosts are the major means of survival of the fungal pathogens. Dissemination (local and long distance) occurs through irrigation water, wind and insects. Effective means of control depends on the knowledge of the various means of survival and dissemination. Fungal pathogens are registered among many orders of fungi, their identification is done by using standard descriptions and keys.
2. PLANT PATHOGENIC BACTERIA
Bacteria are an extremely small, microscopic, unicellular micro-organisms that reproduce by fission. They exhibit three shapes viz, spherical or cocci, rods or bacilli and spiral or spirilli. The improant genera of bacteria that cause diseases in crop plants are:
Xanthomonas, Pseudomonas, Erwinia, Corynebacterium, Agrobacterium and Streptomyces). They enter the plants through wounds or natural openings (stomata) and interfere with physiological functions by destruction of structural integrity by acting upon cell wall components, interference with transport system by mechanical plugging or by producing toxins, interference with host metabolism or by alteration of genetic control mechanisms. These physiological disturbances at the cell level are expressed externally on plant by producing symptoms such as spots, blights, wilts, rots, cankers, galls etc. They are transmitted by a variety of agencies in or on seed and other planting material, soil, wind, rain, insects and nematodes and by cultural practices. The importance of these methods of transmission varies with the host and the pathogen and sometimes more than one method may be involved in any one disease. With regard to control, many seed borne bacterial pathogens can be eliminated either by heat/hot water treatment or by pre-treating the seed with suitable seed dressing chemicals.
3. PLANT PATHOGENIC VIRUSES
Viruses are subicroscopic organisms, multiply only inside living cells and have ability to cause disease. All viruses are parasitic in cells and cause a multitude of diseases to all forms of living organisms. Viruses are much smaller than fungi and bacteria and as such they cannot be visualised under microscope. Viruses can be observed under electronic microscope.
VIRUS TRANSMISSION
Viruses cannot actively penetrate their host cells. Some agency should directly introduce them into the host cell to cause infection. A virus is commonly transferred from an infected host to a non-infected one by a vector, which is often an insect, mite or nematode, but may also be fungus or parasitic flowering plant (e.g. Cuscuta spp, dodder). Sucking insects, such as aphids, leaf and plant hoppers, thrips and whitefly, dominate among the insect vectors, whereas nematodes and fungi play a role in the transmission of soil-borne virus disease. One particular vector may be able to transmit only one virus or many viruses. One particular virus may have one or more vectors. Some viruses may also be transmitted mechanically without the intervention of a vector insect. The sap of an infected plant can reach a healthy one either by means of tools used in cutting and grafting, by rubbing one plant against another.

SYMPTOMS OF VIRUS-INDUCED DISEASES
The symptoms of diseases caused by viruses are numerous and highly variable as their expression is influenced by the condition of the plant and environmental factors such as light, temperature and humidity. Visible symptoms may include local necrotic lesions, mosaic or mottled pattern of lighter and darker green leaf-tissues (chlorosis), yellowing or other discolouration including ring-spots or other distinct patterns, virescence (greening), vein clearing, reduction of plant growth leading to dwarding or stunting, distortion due to unequal growth of cells, excessive tillering, flower and fruit variegations, wilting eventually followed by defoliation and yield depression. A plant may be infected by several viruses at the same time, the symptoms of which may be manifested concurrently and thereby reinforce or mask one another. The study of symptoms may be used for diagnosing the virus disease, but one must take into account that similar symptoms may be caused by different viruses, or even by nutrient deficiencies and genetic disorders. The severity of the symptoms depends on the sensitivity of the plant and virulence of the virus.

HARDENING TRANSPLANTS – VEGETABLE GARDENING

Introduction
The transplanting process can be a shock to rapidly growing seedlings especially when set out into the cold windy garden in the spring. This is especially true for transplants started in the greenhouse, cold frame, hotbed or home. These young seedlings can be made somewhat resistant to heat, cold temperatures, drying and whipping winds, certain types of insect injury, injury from blowing sand and soil particles and low soil moisture by a process termed “hardening.”
The term “hardening” refers to any treatment that results in a firming or hardening of plant tissue. Such a treatment reduces the growth rate, thickens the cuticle and waxy layers, reduces the percentage of freezable water in the plant and often results in a pink color in stems, leaf veins and petioles. Such plants often have smaller and darker green leaves than non hardened plants. Hardening results in an increased level of carbohydrates in the plant permitting a more rapid root development than occurs in non hardened plants.
Cool-season flower and vegetable plants can develop hardiness allowing them to withstand subfreezing temperatures. Unhardened cabbage seedlings have been reported to be damaged by temperatures of -2 degrees C (28 degrees F) while hardened cabbage will tolerate temperatures as low as -6 degrees C (22 degrees F).
Warm-season types of plants even when hardened, will not withstand temperatures much below freezing. If transplanted to the garden or field prior to the average last killing spring frost, such plants should be provided protection by hot caps or other such devices.
Method
Any of the following can be used to harden transplants. A combination of all these techniques at one time is more effective.
1. Gradually reduce water – water lightly at less frequent intervals but do not allow the plants to wilt severely.
2. Expose plants to lower temperature than is reported as optimal for their growth. If biennials are exposed to cold for an extended period, they may bolt in lieu of developing properly. Note: Placing the plants outside during the day to encourage hardening and then bringing the plants back into the warm house during the night often reverses the hardening process. Plants could be placed in a cold frame or other area that does not freeze during the night hours without lose of the hardening process.
3. Do not fertilize, particularly with nitrogen immediately before or during the hardening process. A starter solution or liquid fertilizer could however be applied to the hardened transplants one or two days prior to transplanting into the garden or at the time of transplanting.
4. Gradually expose the plants to more sunlight. This results in the development of a thicker cuticle layer thereby reducing water loss.
Cautions
Hardening is not necessary for all transplants. We recommend that with the exception of tomatoes, plants that are susceptible to frost should not be hardened. Overly hardened plants while withstanding unfavorable outside conditions are slow to get started and may never overcome the stress placed on the plant during the hardening process. We also recommend that plants be hardened for no longer than seven to ten days before planting to the garden site.

CROP ROTATION AND SOIL FERTILITY

Crop rotation is “The practice of alternating the annual crops grown in a specific field in a planned pattern or sequence so that the crops of the same species or family are not grown repeatedly without interruption on the same field”.
-US National Organic Program definition-

OR leaving soil in the best position it can be for continuing/next crops – that includes cover crops, rotations, green manures, catch crops etc.

BENEFITS OF CROP ROTATION
Preventive Pest Management
Crop rotation may limit the growth of populations of agricultural pests including insects, nematodes, and diseases caused by bacteria, viruses, and fungi through regular interruption and replacing crop host species with different plant species that do not serve as hosts. The use of specific crop and cover crop rotations may also be used to control pests through allelopathy, an interference interaction in which a plant releases into the environment a compound that inhibits or stimulates the growth or development of other organisms.
Reduced Weed Competition
Carefully designed crop rotations may also serve to outcompete problematic weed species through shading, competition for nutrients and water, and/or allelopathy.
Distribution of Nutrient Demand Placed On Soil by Crops
Different crops place different nutrient demands on the soil.
Making Efficient Use of Nutrient Inputs
Cropping species that access nutrients from different depths within the soil horizon may make the most efficient use of nutrient inputs. Efficient use of agricultural nutrients may further prevent nutrient losses/leaching and associated environmental pollution.
Nitrogen Fixation
Annual cover crop rotations using nitrogen-fixing (legume) cover crops may contribute significant amounts of nitrogen to succeeding crops as well as adding organic matter to the soil.
Improving Soil Quality
Cover crop rotations allow soils to remain undisturbed for various periods of time during which the processes of soil aggregation can take place. The use of a perennial grass rotation lasting 6 months to one year or more may significantly contribute to organic matter accumulation, stimulate soil biological activity and diversity, and improve soil physical properties.
Increased Crop Yields
The rotation effect – Yield of crops grown in rotation are often higher than those grown in monocultures, even when both systems are supplied with abundant nutrients and water.
Growing a diversity of crops in a given year spreads out labour needs throughout a season. The diversity of crops reduces the economic risks caused by variations in climate and/or market conditions.
TEN BASIC PRACTICES OF CROP ROTATION
Rotate the location of annual crops each year. This is especially true for crops in the Solanaceae family (e.g., peppers, eggplants, tomatoes, potatoes, etc.). Do not follow one crop with a closely related crop species, as pests and diseases are shared by closely related crops. When growing a wide diversity of crops, attempt to group crops into blocks according to the following criteria:
(1) Plant family
(2) Similar timing/maturation periods
(3) Type of crop (i.e., root vs. fruit vs. leaf crop)
(4) Crops with similar cultural requirements (e.g., irrigation, plastic mulch, dry farmed, planted to moisture crops, etc.)
(5) Follow nitrogen-fixing cover crops and/or legume forage crops (e.g., clover, alfalfa) with heavyfeeding crops (e.g., corn) to take advantage of nitrogen supply.
(6) Follow long-term crop rotations (e.g., 1-year perennial rye rotation or pasture rotations) with disease-sensitive crops (e.g., strawberries).
(7) In diverse annual production systems, heavy-feeding crops (crops with high nutrient demands) should be followed by medium-light or shallow-rooted crops, followed by deep-rooted crops.
(8) Always grow some crops that will produce and leave a large amount of residue/biomass that can be incorporated into the soil to help maintain soil organic matter levels.
(9) Grow deep-rooted crops (e.g., sunflower, fava beans, etc.) that may access nutrients from lower soil horizons, alleviate soil compaction, and fracture sub-soil, thus promoting water infiltration and subsequent root penetration.
(10) Use crop sequences known to aid in controlling weeds.
(11) Use crop sequences known to promote healthy crop growth (e.g., corn followed by onions followed by Cole/Brassicaceae crops) and avoid cropping sequences known to promote pests and diseases (e.g., monocultures in general or peas followed by potatoes specifically).
In conclusion, crop rotation is primarily about a cultural system that is based on natural principles. It is about building a fertile living soil and an environment that supports the healthy growth of plants and natural biological control—a situation where synthetic pesticides and fertilizers are unnecessary and even counterproductive.
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