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Thursday, September 15, 2011

Postharvest disease initiation

Postharvest disease initiation needs the flollowing...

      A. The Pathogens
      Upon harvest, ripe fruits and vegetables become subject to attacks of various microorganisms 
      Harvested fruits are rich in moisture and nutrients, which suit the development of pathogens 
      These microorganisms require an injury or damaged tissue to facilitate their penetration
Fungi and bacteria responsible for in-storage decay often originate in the field
These remains in quiescent stages of infection earlier and produce symptoms during storage
Airborne fungi: Cladosporium, Alternaria, Stemphylium, Penicillium, Aspergillus, Rhizopus, Mucor, Botrytis and Fusarium etc.
Soil, irrigation water and plant debris are important source of infection of Botrytis, Sclerotinia and Fusarium etc. 

It is a preliminary stage to fungal penetration. The right environmental temperature, available water or moisture are the most important factors that aid spore germination
Temperature: Most of the storage decay fungi are mesophiles -preferring moderate temperatures around 20-25°C. Shifting the temperature away from the optimum reduces the rate of germination
Water: Water or moisture is essential for fungal spore germination
Nutrient additives: Spores of Penicillium digitatum, the green mould fungus which attacks only citrus fruit, germinate to a minor extent in pure water, whereas the addition of fruit juice greatly accelerates germination

     D. Pathogen Penetration into the Host
1.       Infield penetration and quiescent infections
2.       Penetration through natural inlets
3.       Penetration during and after harvest

1.       Infield penetration and quiescent infections
Harvesting and Picking after Pathogen Penetration
1. Late blight of potatoes (Phytophthora infestans): The infection is caused by the zoospores found in the soil or from infected foliage during harvest. Zoospores penetrate into the tubers through the "eyes", lenticels, growth cracks, wounds, or via the point of attachment to the plant
2. In a warm and damp climate, the blight fungus attack the tomato fruit at its various ripening stages, in the field
Quiescent or Latent Infections
These fungi that penetrate into the host in the field also cause latent or quiescent infection.
1. The gray mould of strawberries (Botrytis cinerea) is a quiescent infection that is acquired in the field
2. The brown rot (Monilinia fructicola) in stone fruits and the decay in apples (Nectria galligena) also originate in the orchard

2.Penetration through natural inlets
Some pathogenic fungi cannot normally penetrate host directly, without the presence of a wound
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 through the stomata

3.Penetration during and after harvest
Penetration via Wounds
Also called  as "wound pathogens“  e.g. Penicillium digitatum and Penicillium italicum conidia in citrus fruits
Rhizopus stolonifer, Alternaria alternata and Monilinia fructicola etc.
Penetration Following Physiological Damage
Physiological damage caused by low temperatures, heat, oxygen shortage or any other environmental stress, increases the fruit susptibility to storage fungi.
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
Penetration Following a Primary Pathogen                  
Several pathogens will enter the host following a primary pathogen "breakthrough".
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.
The sensitivity of melon to the blue-green mold (Penicillium spp.) and to the pink (Trichothecium roseum) is increased during prolonged storage

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