How to Diagnose Postharvest Diseases
Postharvest diseases of apples and pears are caused by a variety of fungi. Some diseases can be easily recognized and separated from others based upon symptoms and signs of pathogens on decayed fruit. However, it can often be difficult to diagnose postharvest diseases based on symptoms alone because different diseases may have very similar symptoms. For example, symptoms of Phacidiopycnis rot in the early stage on d’Anjou pears are very similar to those caused by gray mold. On the other hand, the same disease may also have very different symptoms in the early stage than in the advanced stage. For example, Phacidiopycnis rot appears watersoaked in the early stage of its symptom development but turns black in the advanced stage. Some decay-causing pathogens do not sporulate under cold-storage conditions, increasing the difficulty to confirm diagnosis. The following three-step approach is a practical way for fruit quality control personnel in commercial packinghouses to separate common postharvest diseases, without isolation of causal agents.
The three-step approach involves “Look,” “Feel,” and “Sniff” described as follows:
Look: Look at the appearance of decayed fruit, including the following aspects:
Color of the decayed area
Sign of the pathogen: sporulation or fruiting bodies
Location of infection sites
Presence of wound
Size and shape of the decayed area
Feel: Feel the texture of the decayed area. Postharvest diseases can be divided into two groups:
• soft or watery decay: blue mold and Mucor rot.
• spongy or firm decay: gray mold, Sphaeropsis rot, Phacidiopycnis rot, Bull’s eye rot.
Lesion of blue mold or Mucor rot has a sharp margin between the decayed tissue and healthy tissue. Decayed tissue of blue mold and Mucor rot is completely separable from the healthy tissue, whereas decayed tissue of spongy-type diseases is not separable from the healthy tissue.
Sniff: Decayed fruit of some postharvest diseases produce distinct odors. For example, blue mold produces an earthy, musty odor, whereas Sphaeropsis rot produces a distinct “bandage-like” odor. These characteristics can be used as diagnostic indications of these diseases.
Isolation of causal agents:
Because of similarities in symptoms among postharvest diseases, isolation of causal agents from decayed tissue is often necessary to confirm diagnosis, particularly if no fruiting bodies of the pathogen are present on the decayed fruit for further examination.