We’re at or just before bloom in most of the stone fruit in Ontario.  This is a critical time to provide protection from several fungal diseases.

Blossom blight

The brown rot fungus overwinters in infected mummies in the tree, abscission scars, and cankers resulting from contact with infected fruits the previous year.  Sporulation on these overwintered infected tissues and infection of open blossoms is possible at temperatures as low as 8C but is greater at 15 to 23°C.  Twelve hours of wetting was enough at all temperatures studied (5 to 23°C) for sporulation to occur.  Infection can occur at temperatures as low as 8C with 4 hours of wetness, but shorter wetness periods are required with increasing temperature.  With the warmer weather accompanied by rains, blossom infections are highly likely.

In some research trials I did in the 90’s, we inoculated peach blossoms at pink, balloon, bloom, petal fall and shuck split.  Blossoms were infected only when the stamens and/or stigmas were exposed. In varieties like Babygold 5 with small flowers, these flower parts stick through the blossoms before bloom and are susceptible to infection before full bloom while in varieties with large showy blossoms, like Harrow Beauty, infection didn’t happen until after the petals were open.

Fungicide options for blossom blight management

There are many fungicides registered for blossom blight control in stone fruit.  The important thing to keep in mind is that if the blossoms aren’t open, a protectant fungicide like Fracture, captans or coppers can’t protect the susceptible tissue that isn’t exposed.  That means a fungicide that is locally systemic is your best choice as it will penetrate the petals and protect the fruit from infection.  Historically we have relied on Group 3 fungicides (Bumper, Fitness, Funginex, Indar, Jade, Nova, Quash, Tilt and Topas as well as the Indar component in Aprovia Top and Inspire Super) as the work horses for blossom blight and brown rot management.

A survey of brown rot isolates in Ontario in 2012 found that in 35% of the orchards sampled, populations of M. fructicola had shifted toward reduced sensitivity to Group 3.  Reduced sensitivity does not mean that these fungicides don’t work, they just may not give the same level of control that they used to.  This does, however, emphasize the importance of rotating among groups for resistance management throughout the season.

Locally systemic alternatives to Group 3’s include Rovral, Group 7’s (Fontelis, Kenja and Sercadis), as well as products coformulated with other Groups:  Aprovia Top (Group 3+7), Inspire Super (Group 7+9), Luna Sensation (7+11), Miravis Prime (Group 7+12) and Pristine (Group 7+11).  Rovral is no longer being sold but can be used until April 21, 2021 if you happen to have it in your spray inventory.  Cevya is a new “next generation” Group 3 that showed control comparable to the Group 7’s (Miravis Prime, Luna Sensation and Sercadis) in trials in Michigan when used at the high end of the label rate.  Trifloxystrobin (one of the components of Luna Sensation) also reduced the production of spores on twig cankers and infected blossoms in research trials in Australia.

Black knot

Another thing to consider when deciding on blossom sprays for plums and cherries is whether black knot is a problem in the orchard.  Black knot infections can start as soon as there is enough leaf tissue to hold droplets of water after rain.  The black knot fungus releases spores at a minimum of 11°C, with maximum release at 17°C, 2.5 mm of rain and a wetting period of 24 hours.  Spore release can continue for up to 3 days after a wetting period.  Infection can occur at temperatures as low as 6°C but the optimum temperature is 18-24°C.

The warm wet weather predicted for this week will be highly conducive to black knot infections as well as blossom blight.  For that reason, choosing a fungicide with activity against both blossom blight and brown rot is recommended.  This would include Granuflo-T, the Group 3’s and Fontelis and Pristine.  It’s possible the other Group 7’s might have some activity against black knot, but their activity can be variable for different diseases.

The bottom line:  For peaches, nectarines and plums, if you are targeting just blossom blight, Rovral (if you have it in the spray shed) or one of the Group 3’s is your best bet for blossom sprays.  If black knot is a consideration, Indar or Fontelis is the way to go.

Cherry leaf spot

Cherry leaf spot was more severe than usual in tart cherries and surprisingly severe in sweet cherries last year.  Researchers in Michigan are proposing that the first infections of cherry leaf spot may occur on the tiny bract leaves that subtend cherry blossoms.  The following information and images are excerpts from a 2013 report, Cherry leaf spot: get an early start on protection before the fungus gets started on infection.

There are two main reasons that early infection can result in a fungal epidemic:

1. Once fungal infection occurs in the tree, the fungus will produce tremendous spore numbers from lesions established on the leaves, and in most cases, there are usually more spores developing from lesions than ascospores that are coming up from leaves overwintering on the ground.
2. Spores from lesions on the leaves are much more likely to find new leaf targets within the tree than spores coming up from the ground. The distance from one leaf to another leaf on a tree is minimal, and the potential for spores to infect by moving from one leaf to an adjacent leaf is “easier” than for spores shot from ground level to hit the leaf target up in the tree canopy. This early infection from leaf to leaf can result in a major infection event.

For cherries, you’re looking for a product that will protect from blossom blight and cherry leaf spot plus black knot in tart cherries.  The Michigan researchers recommend that at the bract leaf stage, trees must be covered to prevent infection prior to rain events that could trigger a cherry leaf spot infection. Prior to shuck split, the recommended fungicide for cherry leaf spot management is chlorothalonil (Bravo and Echo). This fungicide is a multi-site protectant and is excellent for leaf spot control and is not at risk for fungicide resistance development. At least two applications of chlorothalonil should be made before shuck split with the goal to minimize the potential of infection at this early timing.  Chlorothalonil will also control blossom blight and black knot.