Black knot update

Wendy McFadden-Smith, Tender Fruit & Grape IPM Specialist, OMAFRA

Don’t let the dry weather this spring lull you into a false sense of security with respect to black knot management!  In most years we observe knots at the base of new shoots, because rainwater sits in the “cup” where the bud joins the shoot and creates ideal conditions for infection.  Shoots are susceptible as soon as there’s enough tissue to hold the water.  In most years, most of the spores are released fairly early in the growing season because we usually have lots of rain so we see most knots developing at the base of new shoots. In a three year study I did with Dr. Northover in the 1990’s in sour cherries, we monitored ascospore release, temperature and rainfall and knot development on trees the following year.  Peak ascospore release occurred from late May through mid- or late June, varying with the year, at sour cherry growth stages from shuck split through the end of shoot growth and fruit maturity.  Large ascospore releases from sour cherry knots followed precipitation of at least 10 mm and a temperature of at least 11°C.

In a year in which the spring was very dry, maximum spore release and shoot infection occurred much later.  Spore release occurred as late as July 14 when sour cherry terminals were more than 25 cm long.   The table below shows the distribution of location of knots the following year:

This means that almost 90% of the infections occurred well after bloom.

As part of a research project, we have been collecting spores from knots.  The highest release of spores occurs from knots after 4 consecutive rounds of 1 hour soaking of knots followed by 4 hours of drying.  We are monitoring spore release from knots as well and no spores have been detected yet.  This means that a whole lot of spores are likely mature and ready to be released when we get a good soaking rain.

 Evaluation of fungicides for management of black knot

As part of a project on black knot funded by Niagara Peninsula Fruit and Vegetable Growers and Ontario Tender Fruit Producers’ Marketing board, I am working with Drs. Deena Errampalli, AAFC, and Jay Subramanian, U of Guelph, to look at various fungicides for management of black knot.  Fungicides (Indar, GranuFlo-T, Bravo, Senator, Fontelis) were applied to single-tree plots in a commercial plum orchard of blue and Japanese varieties with black knot starting at shuck split and continuing weekly for 4 weeks. To test optimum timing for controlling black knot, Bravo was also applied once 14 days after shuck split (14 D post SS), 7 and 14 days after shuck split (7+14 D post SS), at shuck split and 7 and 14 days later (Bravo 3X @ post SS).  Rates of products are given in the table below:

Trees were evaluated for the incidence of black knots early spring 2014.  The graph below shows the incidence of knots per 25 shoots per treatment. (Click the graph to enlarge)

Treatments of Bravo that started after shuck split had more knots/25 shoots than any of the treatments applied 4 times, starting at shuck split.  In 2013, the shuck split period (and possibly earlier) was most important for fungicide protection.  This trial was not done in 2014 but is being repeated 2015-2016.   So far 3 sprays have been applied at weekly intervals starting at bloom.  We’re holding off on the last one until we have a good soaking rain to see whether there’s a difference between the third and fourth spray timing.

Bravo is not registered for use on plum and unfortunately, it does not look as though a label expansion to include plums on the chlorothalonil (Bravo and Echo) is possible.  GranuFlo-T and Indar gave comparable control to Bravo.  Fontelis and Senator were slightly less effective.  Captan/Maestro has also been shown to be effective against black knot.  When you are applying your brown rot sprays before the next rain, it might be a good to use or tank-mix with a product that has activity vs black knot.