Crab-apples looking…..rough?

Fireblight1   Yes the exceptionally cold, wet, cloudy spring has been particularly supportive of fungal and bacterial diseases during leaf emergence this year.  And our ornamental crab-apples are paying the price with thinning canopies and dieback.

Click HERE for the AUDIO version of this blog post.    

Fireblight (Erwinia amylovorais really becoming more noticeable in the landscape on both fruiting and ornamental apple (Malus) and pear (Pyrus) trees.  Look for dark brown to black leaves persisting on twigs, often with a crook in the tip of the twig.  You can also find fireblight on hawthorn (Crataegus), Mountain ash (Sorbus) and Cotoneaster.

Figure 4-154. Infected blooms first appear water soaked and later begin to wilt, shrivel and turn brown or black  Most infections enter the shoot via the flower, killing the developing fruit and then moving further into the twig and then branch. This systemic bacteria can then cause a canker on the branch or main stem, girdling the tree.

FireblightMalus2 Young, succulent shoots can also be the primary infection point and the bacteria spreads into the tree much the same way as it does from floral infections.  If the twig dieback is new, sometimes removing and disposing of the infected twig about 30 cm below the symptomatic tissue can help control the spread of disease.  However, you are knocking bacteria around in the process….and creating another pruning wound…..which may lead to more systemic infections.  In the nursery we recommend bagging, then cutting the juvenile tree off at the base during dry weather with disinfected pruners, and covering up the stump with soil to prevent further spread.

FireBlightCankerIMG_0464.JPG        If the disease is allowed to spread into large branches and the main stem, the resulting canker will kill off sapwood and phloem, resulting in significant dieback and eventual mortality of the tissue beyond. Warm, wet weather favors this bacterial disease and we sure have had our fair share in the last few weeks.

In production, susceptible cultivars of Malus and Pyrus are protected with bactericidal products during bloom and leaf emergence and the weather is used to populate disease prediction models.  This level of protection is no guarantee against fireblight, and rarely available in the landscape. In the landscape, fireblight has the potential to become more damaging as trees are more likely to be pruned, thus creating fresh wound sites for the bacteria to infect and spread inwards towards the trunk.  For this reason, many horticulturalists avoid pruning Malus and Pyrus between May and August. But with storm damage, this may not always be possible. Where pruning must be carried out, growers will decontaminate their pruning sheers with isopropyl alcohol or other sanitizing agents as often as possible between cuts.

OMAFRA Plant Pathologist Mike Celetti reports that there seems to be relatively low incidence of this disease in fruit orchards in 2017 so far.  However, fireblight has been quite severe from 2014 to 2016, meaning that there was still a decent level of inoculum around the last few years.

IMG_1916LookAtSkyAppleScabAnd then there is apple scab (Venturia inaequalis).  A common name that confuses many horticulturalists, the “scab” refers more to the type of lesions that this fungus makes on the fruit. This fungus causes subtle leaf spots along veins and these leaf spots have diffuse margins which makes them difficult to detect.

DSC_0115_01 But the host knows the apple scab fungus is there and dislikes the fungus so much that it responds by forming an abscision zone at the leaf petiole. Causing the leaves to yellow and drop. CODIT in its simplest form.

Apple Frog Eye Leaf Spot 024                       Frog-eye leaf spot (Botryosphaeria obtusa) a.k.a. black rot can also be found on Malus leaves.  This fungal leaf spot is quite obvious, with definitive brown leaf spots occurring between the veins.  It also causes a fruit rot and eventually, branch dieback.  It sporulates on dead wood so pruning out and removing dead wood in apple trees (avoiding May-August) is an important part of disease management.

Image result for crabapple tree ontario                               Crab-apples are a gorgeous member of the Ontario landscape and are much loved (Photo: Malus ‘Prairiefire’  Thank goodness not all crab-apples are susceptible to these diseases and many, many cultivars have good tolerance.  Check out the International Crabapple Society rating system and other references for cultivars and their tolerance to diseases such as fireblight and apple scab.

Click HERE for the AUDIO version of this blog post.    


About Jen Llewellyn

OMAFRA Nursery and Landscape Specialist @onnurserycrops
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