The following points highlight the seven main types of penetration of post-harvest pathogens. The types are: 1. Infield Penetration 2. Latent Infections 3. Contact Infections 4. Penetration through Natural Openings 5. Penetration through Wounds 6. Penetration Owing to Result of Senescence 7. Penetration after Physical Damage.
Type # 1. Infield Penetration:
Fungi and bacteria responsible for in-storage decay often originate in the field or orchard. Late blight of potatoes caused by Phytophthora infestans is an example of decay originating in tuber infection in the field. The infection is caused by the zoospores found in soil or that fall onto the tubers from infected foliage during harvest.
Following germination, the zoospores penetrate into the tubers through eyes, lenticels, growth cracks, wounds, or via the point of attachment to the plant. 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.
In a warm and damp climate, the blight fungus can also attack the tomato fruit at its various ripening stages in the field. The attack usually takes place at the edges of the fruit stalk or directly through the skin. In course of epidemics the entire crop may rot in the field.
However, when the disease is less severe, fruits with no visible symptoms or with slight blemishes might be picked and the fungus will continue to develop during storage.
The brown rot, which develops in citrus fruits during storage, originates in preliminary infections initiated in the orchards by Phytophthora citrophtliora and other Phytophthora species. During the rainy season, the fungal zoospores descend on the lower fruits of the tree and penetrate them directly.
The fungus can develop in the orchard, but zoospore infection occurs a few days prior to picking and the external symptoms are not visible, the fruits will later rot and might comprise a serious problem during storage or transportation.
The principle concern is that the harvest ends fungus development within the host while it is still on the parent plant and causing the decay after harvest or during storage. In many vegetables such as carrot, cucumber, lettuce, celery, cabbage, cauliflower and others, the soft watery rot caused by Sclerotinia sclerotiorum is common.
This fungus, common in the soil might attack the vegetable when still in the field or during harvest and continue to develop in its tissues throughout storage or marketing.
Type # 2. Latent Infections:
Pathogen penetration takes place in the field and will get to storeroom within the host tissue without eliciting any symptoms of decay known as early or quiescent stages of infection. The fungi that penetrate into the host in fields may cause latent or quiescent infections. These pathogens arrive at fruits or vegetables that are still on parent plant.
However, during one of the phases, between their reaching the host and development of progressive disease, the growth is arrested until after the harvest, when physiological and biochemical changes occurring within the host will enable their renewed growth. Verhoeff (1974) has described such arrested infections as “latent infections” in which the pathogen growth is temporarily inhibited.
The latent state of the pathogen, whether involving spores that have landed on the host surface have commenced germination, or primary hyphal development within the host tissues, is linked to a dynamic balance among the host, the pathogen and the environment.
The physiological and biochemical changes occurring in the host tissues after and during storage might affect the pathogen and host interrelationship lead to the activation of the latent pathogen. The brown rot, caused by Monilinia fructicola in stone fruits and the decay caused by Nectria galligena in apples (Swinburne, 1983) originate in the infections of young fruit in orchard.
The Nectria fungus forms a preliminary mycelium within the young tissues, which is the dormant stage of disease, while the interrupted stage in the development of the brown rot caused by Monilinia occur during spore penetration through the stomata or even at the pre-germination stage.
Another example of quiescent infection is anthracnose caused by Colletotrichum gloeosporioides in many tropical and subtropical fruits such as mango,guava, papaya and various citrus fruits. The fungal conidia are found in large quantities on the surface of fruits during their development on the tree.
In presence of free water upon the fruit, the spores germinate and form an appressorium at the tip of the germ tube. The appressoria function as resting spores on the fruit, since they resist environmental conditions much better than the conidia and remain vital for long periods while embedded in the natural wax of the fruit or bound to its surface.
Type # 3. Contact Infections:
Fruits or vegetables which are free of a pathogen invasion by any means of penetration might be infected through contact with infected produce during storage. Contact infection is a significant factor in spreading white watery rot (Sclerotinia spp.) and bacterial soft rot (Erwinia spp.) in lettuce, carrot, cabbage, celery or squash during storage.
The development of Botrytis infection in stored strawberries, which turns into “mummies” covered with a gray layer of spore-bearing mycelium, causes contact infection and makes fruits susceptible to the disease. Similarly, one strawberry or tomato or a single grape infected by Rhizopus can spread the infection within the container.
In fact, contact infection by Botrytis or Rhizopus is typical of many fruits and vegetables and account for major losses caused by these pathogens during long-term storage. In citrus fruits, contact infection of green and blue mould caused by Penicillium digitatum and P. italicum is very common; it often occurs during shipment and under certain conditions disqualifies the entire shipment.
Type # 4. Penetration through Natural Openings:
Some pathogenic fungi and bacteria that cannot normally penetrate the directly, without the presence of wound in its surface, can penetrate through natural openings such as stomata and lenticels.
The penetration of germ tubes of Colletotrichum gloeosporioides spores into young papaya fruits and the penetration of Monilinia fructicola spore germ tubes into young stone fruits can take place through stomata while the fruit is still in the orchard.
Penetration through lenticels has also been described for Alternaria alternata spores in mango and Gloeosporium perennans in apples. The lenticels in potato tubers are prone to develop bacterial soft rot during storage. Most of lenticels are infested with the cells of Erwinia cartovora during harvest.
The bacteria remain inactive in the lenticels until the favourable conditions such as mechanical pressure, existence of free water or low oxygen pressure within the tuber enhancing the tuber sensitivity to decay.
Type # 5. Penetration through Wounds:
In contrast to pathogens that attack the fruit and vegetable in the field, most of the storage pathogens are incapable of penetrating directly through cuticle or epidermis of the host, but require a wound or an injury to facilitate their penetration. Therefore, the fungi and bacteria that develop during storage are often called wound pathogens.
Growth cracks present on harvested fruits and vegetables are natural avenues of infections. Careless separation of the fruit or vegetable from the parent plant might result in an injury liable to be attacked by the pathogen. The extent of injury caused by mechanical harvesting is far greater than that caused by a manual operation.
The harvesting is accompanied by mechanical injuries enables the weak pathogens to penetrate. It has turned out that the depth of the injury, combined with the humid conditions during storage, determine the fate and extent of the infection.
Each scratch, incision or other mechanical injuries inflicted on the fruit or vegetable during handling, harvesting, transporting, sorting, packing and storing might present adequate penetration points for the storage pathogens. A possible penetration point is the stem end separation area where damage often occurs during fruit picking and in this regard, the separation area is no different from any other injury.
It regularly happens that simultaneously with the injury, a large amount of fungal spores and bacterial cells arrive at the injured area, some of which use the injury site to penetrate and infect the host. Penetration through wounds is a characteristic of Penicillium digitatum and Penicillium italicum pathogen conidia in citrus fruits.
Wound infection is also characteristic of Rhizopus stolonifer that causes watery soft rot in many species of fruits and vegetables; of Alternaria alternata that causes a dark, rather dry decay in a large number of fruits and vegetables stored for a prolonged period; of Geotriclium candidum which causes the sour rot in citrus, melons and tomato fruits; and of various Aspergillus, Cladosporium and Trichothecium species, and other storage fungi of various fruits and vegetables.
Type # 6. Penetration Owing to Result of Senescence:
Tissue senescence during continuing storage reduces disease resistance. Commonly at the end of storage period, the sensitivity of melons to blue green mould caused by various species of Penicillium and pink mould caused by Trichothecium roseum is increased. A senescencing onion that has commenced sprouting often harbours base decay, caused by various species of Fusarium.
Generally, the rate of decay during storage increases with the duration of storage as tissue senescence progresses. Increasing the tissue sensitivity to diseases during storage also contributes to contact infection of a healthy product by an infected one covered with spore-bearing mycelium.
Type # 7. Penetration after Physical Damage:
Injuries caused by low temperature, heat, oxygen shortage or any other environmental stress increases the fruit or vegetable sensitivity to storage fungi. The physiological damage can be externally expressed through tissue browning and splitting, thus forming site vulnerable to invasion of wound pathogens.
Extreme environmental conditions enhance sensitivity to an attack without any visible external signs of damage. Development of sun scald lesions in apples leads to Alternaria alternata and Stemphylium hotryosum attack while Alternaria may also be associated with other physiological disorders on apples such as bitter pit or soft scald.