Apricots Diseases Nectarines Peaches Plums Uncategorized

See Spots Run: Bacterial spot and peach scab season is here.

Bacterial spot symptoms have started to become apparent in apricot, peach, nectarine and plum blocks following the warm rainy weather we’ve experienced in Ontario recently. 

Bacterial Spot

The bacteria that cause bacterial spot overwinter in buds, protected areas on the woody surface of the tree (e.g., cracks in the bark), and in leaf scars that became infected during leaf drop the previous season.

Blackened dead shoot surrounded by leaves with symptoms of bacterial spot.
Arrow points to blackened twig that showed up in spring to spread bacteria to start epidemic. (Photo source: https://www.canr.msu.edu/ipm/agriculture/fruit/bacterial_spot_of_peach_and_plum)

Leaf scar infections usually develop into spring cankers.  The bacteria are spread from cankers in dripping dew and in splashing and/or wind-blown rain to the newly emerging leaves. They can also infect through natural openings or wounds in leaves and fruit.

Optimal temperature for bacterial multiplication in leaves and for infection of leaves and fruit ranges from 20 to 30ºC. Leaf wetness is necessary for pathogen infection: from 3 to 6 h of wetness is enough to cause high disease severity at optimal temperatures.

Bacterial spot multiplication in leaves is favored by wet conditions that cause water congestion in plant tissue. Rains, dew and high relative humidity assist bacterial entry into plants and multiplication. Wetness in the morning is not a optimal for infection as wetness that lasts a day or more. This sensitivity to wetness is evident in the symptoms. Tips of leaves, where moisture evaporates more slowly, show the spot lesions and chlorosis more quickly than the base of leaves.

Spread and entry of the bacterial spot pathogen into plants is favored by abrasions and nicks caused by blowing sand, especially on outside peach rows adjacent to field roads. High wind speeds from sprayers may also help to spread bacterial spot. 

Peach leaves with brown lesions limited by veins or concentrated at leaf tips.
Bacterial spot lesions with brown centres and yellow ring.
Peach leaves with holes where centres of bacterial spot lesions have died.
Shot hole symptom. Centres of bacterial spot lesions turn brown and drop out.
Photo credit: Jiosey Caputo, Niagara Pest Scouting Club
Plum leaves with dark spots of bacterial spot.
Bacterial spot lesions on plum leaves.

Fruit are very susceptible to infection as soon as they are exposed at shuck split through pit hardening but new infections can develop until harvest. Severe fruit infections are more common when frequent periods of rainfall or even extended heavy dews and very high humidity occur from late bloom to near pit-hardening.

Cueva (copper octonoate) is the only product labeled for bacterial spot in stone fruit.  Use at 1% v/v in 470-940 L/ha.  Do not apply at less than 7-day intervals and avoid applying copper under slow drying conditions.  This is to avoid the accumulation of toxic copper ions.  There will always be some injury resulting from copper applications to peach and nectarine; the goal is to make sure the cure is not worse than the disease. Differentiating between bacterial spot and copper phytotoxicity can be challenging. The image and table below provide some differences.

Contrasting images of bacterial spot on left and copper injury on right.  Table below explains differences.
Bacterial Spot
Copper injury
Angular lesions
Round lesions
Lesions always bordered by the veins
Lesions randomly located, not limited by veins
Few or many lesions on a leaf
A lot of holes
Yellowing associated with lesions
Yellowing not associated with lesions
Leaf will always turn yellow and fall off (can occur with only a few lesions)
Leaf drops only with severe damage
Photo and Table Source: http://ipm.uconn.edu/documents/raw2/html/816.php?aid=816

Timing of applications is critical. Sprays should be applied prior to rainfall but with enough time for the product to dry.  Slow drying conditions will increase the probability of injury to leaves by copper sprays as will repeated applications. With continued use of copper, the older leaves will often develop a buildup of copper if rain does not wash off copper in between sprays.

Dr. Kari, Peter, Penn State University, achieved bacterial spot control using Serenade Opti (1 kg/ha) in rotation with copper; the results were similar to when they used copper for every spray in the season.

Peach scab

Peach scab can be cause yield losses in peaches, nectarines and apricots.  The fungus that causes it overwinters in lesions on twigs. Conidia are produced in the spring after petal fall and are windblown or splashed by rain.  During the spring and summer, conidia are produced when relative humidity is at least 100% for 24 hours, and temperatures exceed 16°C. The conidia (spores) are spread by wind or by rain splash.  Conidial production on 1-year-old twig lesions commences at bloom and continues for approximately 10 weeks, tapering off during early to mid-summer.

One year old peach shoot at fruit set showing darkened lesions around leaf scars.
Cankers on one-year-old twigs infected last year act as the source of scab spores.

Although foliar lesions have been reported in the literature, they are very infrequently observed so one of the main ways to differentiate between peach scab and bacterial spot is the absence of foliar symptoms in scab.

Epidemics occur most readily when conditions are humid, rainfalls are frequent, and temperatures range between 18 and 24°C. However, infection can still occur as low as 4-7°C and as high as 32-35°C. Since fungal spores are spread primarily by rain splash, it is common for clusters of small lesions to develop at the top of the fruit near the stem where rain most frequently contacts the fruit. 

Peaches are most susceptible during the shuck-split stage of growth. Because they lack fuzz, nectarine fruit can be infected earlier than peaches:  1–2 weeks after petal fall.  Although fruit may become infected shortly after shuck split and continue to be susceptible to infection throughout the growing season, symptoms are typically not visible on fruit for six to ten weeks so only those infections which are initiated between shuck split and six weeks before maturity will exhibit symptoms before harvest.

It is critical to get some fungicide protection in place in blocks with a history of scab before the next rain event. A scan of the literature shows that the most effective products to manage peach scab include captan, Senator, Flint, Pristine, Inspire Super, Luna Sensation, Syllit and Sercadis.  Fontelis, Group 3 fungicides and sulphurs will also give some protection.  Be aware of the extended restricted entry intervals relative to fruit thinning if you use the WSP formulations of captan.  WDG formulations can be used according to the existing label for this season only.

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