Occurrence and importance
Jonathan spot may occur on susceptible varieties wherever apples are grown. It is occasionally found on ripe apples before picking or on drops, but Jonathan spot is primarily a disease that occurs during storage or transit or on the market. The disorder was first described on Jonathan and is perhaps most common on that variety. Other varieties, however, such as Wealthy, Yellow Newtown, Grimes Golden, Rome Beauty, Gravenstein, Northern Spy, Winter Banana, and Golden Delicious may be affected.
In reporting the disorder, especially on varieties other than Jonathan, there has been a tendency to introduce other names such as "Spy spot," "physiological spot," and "senescent spot" as synonyms of Jonathan spot.
The disease is important chiefly because of its effect on the appearance of the fruits. Its most serious characteristic is a tendency to develop in transit or storage to such an extent that marked damage results to fruits that were apparently in good condition when shipped or stored.
In general, decay following Jonathan spot is not a problem, but under favorable conditions for decay, Alternaria tenuis or some other weak pathogen could become established in the dead tissues.
Jonathan spot originates at the lenticels. The pattern of affected tissues may vary as much within certain varieties as it does among them. Two distinct types of spots may occur on the Jonathan variety. One is a sharply sunken area at the lenticel that is very dark brown or black and about 1/8 of an inch in diameter (top photo). These spots penetrate the flesh somewhat below the surface of the skin. This is deeper than other types observed. Just as common on Jonathan, however, as the above type are moderately light brown, irregularly shaped or somewhat lobed areas of skin that surround the affected lenticel and may cover an area up to 1/4 inch in diameter (second photo). Affected lenticels are darker than the surrounding brown skin and are only slightly sunken. The surrounding brown skin may be slightly sunken, depending on how long the condition has been present.
Next to Jonathan, Rome Beauty is about as susceptible to Jonathan spot as any variety on the market. A full range of symptoms has been observed on Rome Beauty. Some fruits showed only a very small sunken area including little more than the lenticel, surrounded by normal skin or a very narrow band of brown skin (third photo). One of the most typical symptoms observed was that of numerous but isolated dead, brown lenticels surrounded by round or irregular shaped areas of brown skin (fourth photo). Often only the lenticel was sunken. In the most severely affected fruits, the discolored lenticels were so near together that the surrounding brown skin coalesced, making a large brown area on the skin.
On Golden Delicious only one type of Jonathan spot has been observed. Affected lenticels, surrounded by a narrow band of dead skin, were sharply sunken and brown (fifth photo).
Jonathan spot tends to occur on the blush side and occurs more at the stem half or around the middle of the apple than at the calyx end. The spots may be sparse or numerous.
Generally Jonathan spot discolorations can be removed by thinly peeling the fruit. This characteristic readily separates it from bitter pit. Even if a small portion of Jonathan spot remained it would be wet and not dry and spongy as in bitter pit.
Jonathan spot is a physiological disease, but its exact cause is unknown.
The disease is common after a dry season. It is usually worse on large apples than on small ones. It is worse on late-harvested fruits than on those harvested in prime maturity. Jonathan spot is also worse on fruits cooled slowly or those stored at temperatures somewhat higher than temperatures recommended for the variety.
Jonathan spot appears to be induced by some abnormal metabolism associated with the senescence of susceptible apples.
Harvesting fruits at optimum maturity, prompt cooling, and sharp control of storage temperatures reduces aging and delays the onset of Jonathan spot. Storage in controlled atmospheres of 2.5 to 5.0 percent carbon dioxide and 3.0 percent oxygen controls the disorder on Jonathan apples.
Jonathan spot, sunken, on Jonathan
Jonathan spot, superficial, on Jonathan
Jonathan spot, Rome Beauty
Jonathan spot, Rome Beauty
Jonathan spot, Golden Delicious