Brown rot (Monilinia)

Symptoms

On pome and stone fruit, brown, enlarging rot spots appear, on which concentrically arranged, grey-beige cushions of spores form. Infected fruit rot, dry out and mummify, often remaining on the tree until the next season. In spring the fungus also infects blossoms and short shoots, causing them to die back and killing the branch tips. The disease has two distinct phases. Spring blossom and twig blight (most often Monilinia laxa on stone fruit — sour cherry, sweet cherry, plum, apricot) begins on the blossoms, which turn brown, wilt and dry out without dropping; the infection enters through the stigma and style into the short shoot, girdles it and causes whole shoots to die back, with withered leaves hanging as if scorched — hence the misleading resemblance to frost damage. On stone fruit, infected shoots often exude gum (gummosis) and develop small, cracked bark cankers. The summer-to-autumn brown rot of the fruit (mainly Monilinia fructigena on apple and pear) develops on ripening fruit: the rot spot quickly engulfs the whole fruit, the flesh turns brown and softens, and characteristic, concentrically arranged rings of cream-beige cushions of conidial spores grow out on the surface. In cool conditions (including in storage), the fruit may take the form of hard, black glossy mummies lacking sporulation (so-called black rot). Mummified fruit hanging on the tree and Monilinia cankers on the shoots are the main source of spores in the following season.

Treatment

Cutting out and removing infected shoots and collecting and destroying the mummified fruit, which are the main source of infection. Fungicide sprays are used during flowering to protect the blossom and before harvest; it is also important to avoid injuring the fruit, because wounds are entry points for infection. Sanitary pruning is done with a margin — the shoot is cut several to a dozen or so centimetres below the visible edge of the blight, in dry weather, and the tools are disinfected between trees. Mummies are collected throughout the season, including those fallen to the ground, and taken away or dug in deeply (not onto the home compost). Chemical protection is most effective preventively: the first treatments during flowering (at full bloom and towards the end of flowering) protect stone-fruit blossoms from blight, while pre-harvest treatments — observing the pre-harvest interval — limit fruit rot. Systemic and translaminar fungicides are used (including from the triazole, anilinopyrimidine, SDHI and strobilurin groups) together with contact products; substances from different modes of action must be rotated, because Monilinia quickly develops resistance to a single group. Harvesting is done carefully, avoiding abrasions and punctures, and the fruit is cooled quickly after picking — low temperature strongly slows the development of rot in storage. Biological products based on antagonistic microorganisms (e.g. Bacillus subtilis, yeasts of the genus Aureobasidium) can support protection, especially around harvest, but do not replace sanitation or treatments at critical times.

Prevention

Thinning the crowns for better ventilation, curbing pests and diseases that damage the fruit skin (codling moth, scab), harvesting undamaged fruit and removing the mummified fruit in autumn and winter. The foundation is breaking the disease cycle: rigorously removing all mummies and cankered shoots during winter pruning deprives the fungus of its spore base in spring. A loose, well-thinned crown dries faster after rain and dew, which limits blossom infection. It is worth thinning the fruitlets so that ripening fruit do not touch — direct contact of a diseased fruit with a healthy one is one of the main routes by which the rot spreads. Balanced fertilisation (without excess nitrogen, which produces soft, susceptible tissue) and consistent control of codling moth, wasps, birds and scab limit the wounds that serve as entry points for infection. On sites with a history of the disease, choosing less susceptible cultivars, harvesting in dry weather and cooling the crop quickly all help. Preventive and sanitary methods work best together — on their own, without protection in years with wet flowering, they may not be enough.

How to tell it apart from look-alikes

Control — organic and chemical methods

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