Sunday, 20 October 2019

Post harvest Pathogens Disease Initiation

Post harvest Disease Initiation


THE PATHOGENS


Upon harvest, ripe fruits and vegetables become subject to attacks of various microorganisms incapable of attacking earlier in the course of growth in the field. These are largely weak pathogens, fungi, and bacteria, typical of the harvested and stored fruits and vegetables. Disease resistance, which was embodied in the plant organ designated for storage during its developmental stages on the plant, weakens as a result of separation from the parent plant. In addition, picked fruits and vegetables are rich in moisture and nutrients, which suit the development of pathogens. Upon ripening the fruits and vegetables often become more susceptible to injury and, therefore, more susceptible to the attack of those microorganisms that require an injury or damaged tissue to facilitate their penetration (Eckert, 1975). Moreover, during the prolonged storage of fruits and vegetables, a series of physiological processes occur which leads to the senescence of the vegetal tissues and, in parallel, to their increased susceptibility to weak pathogens that attack senescence vegetable tissues.

 THE ORIGIN OF THE PATHOGENS


Fungi and bacteria responsible in-storage decay often originate in the field or orchard. When penetration into the host takes place in the field, the pathogen, which is then in its early or quiescent stages of infection, will get to the storeroom within the host tissue eliciting any symptoms of decay. Yet, even when the preharvest infection has not taken place, there are always fungal spores and bacterial cells, which are typical components of the airborne microorganism population, on the fruit and the vegetable during their growth. This cargo of spores and cells is transferred to storage with the harvested fruits and vegetables. Examination of the fungal spore population on the surface of stems, leaves, flower parts, fruits, and other plant organs, after harvest, reveals the presence of many important airborne fungi such as species of Cladosporium,  Alternaria,  Stemphylium,  Penicillium,  Aspergillus, Rhizopus,  Mucor, Botrytis, Fusarium, and others. Many of the airborne fungi are among the most important decay agents which affect harvested fruits and vegetables and, given the right conditions, develop and cause decay.

Many postharvest pathogens perpetuate on crop debris in the field and can, under suitable conditions, develop and produce abundant new spores. These fungal spores are easily carried by air currents, winds and rain, or dispersed by insects, to the flowers and the young fruits, at various stages of development, and form a potential source of infection. Soil, irrigation water and plant debris form an important source of infection of various vegetables. Soil-residing fungi and bacteria can attack the bulb, tuber, root and other vegetal parts, while these are still attached to the parent plant, through a tight contact with the soil, by lifting of soil particles by winds, rains or irrigation, through growth, or by arriving in storage with soil residues attached to the vegetable. Some soil microorganisms, such as species of the fungi Botrytis, Sclerotinia and Fusarium or the bacterium Erwinia carotovora, are among the main decay agents in stored vegetables. Host infection can occur preharvest, during harvest or during any of the postharvest handling stages.

Harvesting instruments,  containers, packing houses with their installations, the hands of the packers or selectors, the atmosphere of the storage rooms - are all bountiful sources of fungal spores. Despite the diversity of microorganisms that the fruits and vegetables carry with them into storage, only a few species can naturally attack them, while other species that reside on the surface, at times in large quantities, will not penetrate and will not cause any decay. The development of disease during storage depends, primarily, on the existence of the appropriate microorganisms alongside a given host. However, in order for the fungal spores or bacterial cells that have reached the suitable host to be capable of infecting, they have to encounter the appropriate conditions for germination on the surface of the host, to penetrate into the host tissues and to develop there.

SPORE GERMINATION


SPORE GERMINATION


Spore germination is a preliminary stage of fungal penetration into the host. Environmental temperature, available moisture and, sometimes, the presence of nutrients transferred from the host into the water, are the most important environmental factors that aid spore germination.
There are describe 3 main factors that effect of spore Germination:

  • Temperature.
  • Water and
  • Nutrient additives.


TEMPERATURE 


The temperature is the most important factor for fungal spore germination. The optimal temperature for the germination of fungal spores is 20-25°C, although the temperature range allowing germination can be much wider. The further from the optimal temperature is a long time until the initiation of spore germination - a phenomenon that affects the prolongation of the incubation period of the disease. Shifting the temperature away from the optimum reduces the rate of germination and retards germ tube elongation. It the temperature sufficiently low then inhibit spore germination altogether and thus constitute a disease-limiting factor.

WATER


Water or moisture is essential for fungal spore germination, although spores of some fungi are capable of germination if there is a very high relative humidity of the surrounding air (Roberts and Boothroyd, 1984). Some of the spores mostly fruit and vegetable pathogens, can germinate in pure water or water with low nutrient concentrations, transferred from the hot surface to the water by osmosis or supplied to the spores by battered cells and injured in the wound region, the typical court of infection for many postharvest pathogens.

NUTRIENT ADDITIVES


The green mold fungus (Penicillium digitatum) spores ar effect only citrus species fruits, germinate to a minor extent in pure water, whereas the addition of fruit juice greatly accelerates germination (Pelser and Eckert, 1977). Spore germination testing the effect of the juice components revealed that of the sugars within the juice (glucose, sucrose, and fructose), glucose is the best stimulant. The ascorbic acid is stimulated by enhanced germination, whereas the citric acid has no stimulating effect. The combination of ascorbic acid and glucose results in a germination rate quite close to that stimulated by the whole juice.

PATHOGEN PENETRATION INTO THE HOST


PATHOGEN PENETRATION


Postharvest pathogens can be divided, according to the timing of their penetration into the host, into those that penetrate the fruits or vegetables while still in the field but develop in their tissues only after harvest, during storage or marketing, and those that initiate penetration during or after harvest.

1. INFIELD PENETRATION AND QUIESCENT INFECTIONS


Harvesting and Picking after Pathogen Penetration

Phytophthora infestans late blight is an example of decay originating in tuber infection in the field. The infection is caused by the zoospores found in the soil or that fall onto the tubers from infected foliage during harvest.  Following germination of Phytophthora infestans zoospores penetrate into the tubers through the eyes, lenticels, growth cracks, wounds, or via the point of attachment to the plant (the stolon). Potato tubers that were infected a few days prior to harvest or during the harvest itself are brought to storage carrying the disease in its early developmental stages, with no visible symptoms of decay. These potato tubers will decay while stored under high humidity and at a temperature of over 5°C.

Quiescent/Latent Infections

The fungi that penetrate into the host in the field also include pathogens that cause latent or quiescent infection. These pathogens reach fruits or vegetables that are still on the parent plant. However, during one of the phases between their reaching the host and the development of the progressive disease, their growth is arrested until after the harvest, when biochemical and physiological changes occurring within the host will enable their renewed growth. Verhoeff (1974) has described such arrested infections as "latent infections". However, since this term has been used for describing different phenomena in various areas, it was later suggested by Swinburne (1983) to leave the term "latent infection" for wider uses and adopt "quiescent infection" for cases in which the pathogen growth is temporarily inhibited. In some instances, we find that the inactive state is termed 'latent' if not visible to the eye and 'quiescent' if visible, while in others the two terms are frequently used to describe the same phenomenon. Jarvis (1994) believes that the latent state of the pathogen, whether involving spores that have landed on the host surface, spores that have commenced germinating, or primary hyphal development within the host tissues, is linked to a dynamic balance among the host, the pathogen and the environment.

Nutritional and Energetic Requirements of the Pathogen

 The first theory claims that young unripe fruit does not provide the pathogen with the nutrition and energy required for its development. Since, during ripening, a conversion of insoluble carbohydrates to soluble sugars occurs, it is no wonder that the resistance to rotting of the young fruit has been attributed, in several cases at least, to the sugar content of the tissues. This theory complies with the results of experiments conducted on apples, in which an artificial increase of the sugar level, by the addition of sugar-regulated compounds (such as 2,4-dinitrophenol) to the fruit, had accelerated the decay caused by Botryosphaeria ribs. Yet, since these compounds also accelerated the onset of the climacteric peak of the fruit respiration, their stimulating effect cannot be attributed to the sugar level alone. In addition, the accelerated onset of the susceptibility of apple fruits to the pathogen could also be attributed to the reduction in the toxicity of antifungal compounds in the presence of sugars. The fact that fungi which normally attack ripe fruit may also develop on extracts prepared from unripe fruit, attests to a lack of a clear link between resistance and the shortage of available nutrients in the young fruit.

Activation of Pathogen Enzymes

The second theory suggests that the unripe fruit does not supply the pathogen with compounds that induce the formation and activity of cell-wall degrading pectolytic enzymes. In addition, in cases whereby the fungus produces cell-wall degrading enzymes, cross-linking of the cell-wall pectic compounds might block the access of these enzymes to the sites within the cell wall. Furthermore, enzymes may be inactivated by inhibitors present in higher quantities in immature fruits than in mature fruits (see the chapter on Host Protection and Defense Mechanisms - Inhibitors of Cell-Wall Degrading Enzymes).

The Induction of Antifungal Compounds in the Host

The third and fourth theories point at a relation between the presence or formation of anti-fungal compounds in the young tissues and the creation of quiescent infections. These antifungal compounds may be:
(a) preformed compounds, the existence of which is not pathogen-related;
(b) The host preformed compounds further induce, either in the same tissue it was originally formed or in a new tissue; 
(c) phytoalexins (post-formed compounds) induced by the host as a counterattack against the pathogen.

2. PENETRATION THROUGH NATURAL INLETS


Some pathogenic fungi and bacteria that cannot normally penetrate the sound host directly, without the presence of a wound on its surface, can penetrate through natural openings such as stomata and lenticels. The Colletotrichum gloeosporioides spore germ tubes penetration into young papaya fruits and the penetration of Monilinia fructicola spore germ tubes into young stone fruits can take place through the stomata while the fruit is still in the orchard.


3. PENETRATION DURING AND AFTER HARVEST


Penetration via Wounds

Most of the pathogens that attack the fruit and vegetable in the field. some of the storage, pathogens are incapable of penetrating directly through the cuticle or epidermis of the host but require a wound or an injury to facilitate their penetration. Therefore, the bacteria and fungi that develop during storage are often called wound pathogens. The wound can vary in nature. Growth cracks present on harvested fruits and vegetables are natural avenues of infection. The actual harvesting is accompanied by mechanical injuries that enable the weak pathogens to penetrate. The extent of injury caused by mechanical harvesting is far greater than that caused by manual operation (Fuchs et al., 1984). Each scratch, incision, blow or other mechanical injury inflicted on the fruit or vegetable during each of the handling processes - harvesting, gathering, transporting, sorting, packing and storing - might present adequate penetration points for the storage pathogens. A likely penetration point is the stem-end separation area, where damage often occurs during fruit picking.

Penetration Following Physiological Damage

Physiological damage caused by low temperatures, heat, oxygen shortage or any other environmental stress, increases the fruit or vegetable sensitivity and exposes it to storage fungi. The physiological damage can be externally expressed through tissue browning and splitting, thus forming locations vulnerable to the invasion of wound pathogens. Yet extreme  environmental conditions might enhance sensitivity to an attack without any visible external signs of damage. A tomato fruit exposed to chilling temperatures or heat treatments is liable to be attacked by Botrytis cinerea even when there are no visible symptoms of damage. Exposure to extreme temperatures also partially broke the resistance of previously resistant tomato genotypes {nor and rin fruits and their hybrids) both to B, cinerea and to Alternaria alternata. Alternaria rot develops typically also in zucchini, following chilling injury, whereas cucumbers and melons exposed to excessively low temperatures are sensitive to various Penicillium and Cladosporium species, respectively. An alternata and Stemphylium botryose also tend to attack apple fruits following the development of sunscald lesions, while Alternaria may also be associated with other physiological disorders on apples such as a bitter pit or soft scald.

Penetration Following a Primary Pathogen

Several pathogens will enter the host following breakthrough a primary pathogen. Sometimes, nature deploys a sequence of pathogens. Phytophthora (late blight of potato) infect with the development of the secondary bacteria, the primary decay, which is hard in nature, turns into a soft decay as a result of the bacterial enzymatic activity on cell walls.  Penicillium expansum can enter the apple fruits following their infection by the fungi Mucor, Gloeosporium and Phytophthora.

Penetration due to Tissue Senescence

Tissue senescence during prolonged storage also reduces disease resistance. Thus, at the end of the storage period, the sensitivity of melon to the blue-green mold caused by various species of Penicillium and to the pink mold caused by Trichothecium roseum is increased (Barkai-Golan, unpublished). A sentencing onion that has commenced sprouting often harbors base decay, caused by various species of Fusarium. Generally, during storage increases the rate of decay with the duration of storage as tissue senescence progresses. Increasing the tissue sensitivity to diseases during storage also contributes to the contact-infection of a healthy product by an infected one covered with spore-bearing mycelium.

Contact Infection

Fruits or vegetables that were spared a pathogen invasion via any of the means of penetration might still be infected during actual storage, through contact with infected produce. Contact-infection is a significant factor in the spreading of white watery rot (Sclerotinia spp.) and bacterial soft rot (Erwinia spp.) in lettuce, cabbage, celery, carrot or squash during storage.  The development of Botrytis in stored strawberries, which it turns into "mummies" covered with a gray layer of spore-bearing mycelium, causes a "chain" contact-infection and jeopardizes the entire basketful of fruit. Similarly, one strawberry or tomato, or a single grape berry infected by Rhizopus constitutes a focus from which the decay can spread within the container when it is transferred from refrigeration to shelf conditions.  In fact, contact-infection by Botrytis or Rhizopus is typical of many fruits and vegetables and may account for the major losses caused by these pathogens during long-term storage. In citrus fruits, contact-infection by the green and blue mold rots {Penicillium digitatum and P. italicum) is very common; it often occurs during shipment and can, under certain conditions, disqualify the entire shipment.

No comments:

Post a comment