Stroby still best for black spot and mildew

Pome fruit Oct. 12, 2020

Long-term favourite of Australian pome fruit growers, the strobilurin fungicide Stroby – was the first of its class to be registered in Australia.

Stroby WG is just one of the many niche products that have been picked up by Australian Agribusiness – through its agriculture division Barmac – to continue supplying Australian specialty markets that have become too small for larger companies to service directly.

Australian Agribusiness technical manager, Dr Brett Morris said BASF still owns the Stroby brand, but Barmac is now distributing Stroby on their behalf to maintain this key fungicide for Australian apple and pear growers’ spray programs.

Many years down the track, Stroby remains an important inclusion in spray programs for the major diseases, blackspot (scab) in apples and pears, and powdery mildew in apples.

“Stroby WG is a protective and curative fungicide containing 500 g/kg kresoxim-methyl. It is recommended for IPM programs, with its low toxicity for beneficial organisms including bees, predatory mites and earthworms,” Dr Morris said.

“Being a Group K fungicide, it also provides an alternative for fungicide-resistance management. Stroby has excellent protectant activity against both leaf and fruit scab in apples (Venturia inaequalis) and pears (Venturia pirina) and against powdery mildew (Podosphaera leucotricha) in apples – leading to increased production of quality fruits.”

Scab is the most serious and widespread fungal disease of apples and pears, infecting leaves, shoots, buds, blossoms and fruit, especially in regions with high rainfall and relative humidity during the growing season.

The scab fungus is spread by wind and rain, and throughout the growing season lives and grows on the tree, producing vast numbers of spores. The fungus survives the dormant season on the orchard floor in leaves that have fallen in autumn.

In a season favourable to scab infection, Dr Morris said the disease is easier to control in a clean orchard than in a heavily infected one. Pruning to increase air flow will also aid canopy drying and improve spray penetration.

“At present, the basis for effective scab control is the use of protectant and post-infection sprays during spring, and orchard sanitation practices to reduce scab carryover into the next season.

“Protective spraying keeps the susceptible parts of the tree covered with fungicide to prevent primary infection, killing spores before they can establish an infection.”

Protectant sprays of fungicides, including Stroby WG may commence at spurburst, with sprays at regular intervals afterwards to ensure that rapidly-developing leaves and fruits are covered with a fungicide residue that will prevent spores from germinating.

“Control by protectants may need supplementing with curative fungicide after infection.

Similarly, infection periods leading up to harvest may mean extra sprays (up to six weeks before harvest) to prevent the serious problem of scab development on fruit in storage,” Dr Morris said.

Powdery mildew in apples is caused by the fungus Podosphaera leucotricha overwintering in fruit and leaf buds, affecting leaves, buds, shoots and fruits. Many currently popular apple varieties are highly susceptible to powdery mildew, which unlike apple scab, is favoured by dry conditions.

Dormant shoots that were heavily infected in the previous growing season will be covered with dense white mycelium, and the terminal bud appears pinched and shrivelled.

Early fruit infection causes a web-like russet on the skin that may be difficult to distinguish from early spray damage.

Dr Morris said there is no adequate substitute for a series of spring and summer sprays, commencing at early pink stage, to prevent powdery mildew infection of fruit and leaf buds.

“Stroby WG is registered for powdery mildew in apples, as well as scab in apples and pears.

“It’s highly rainfast once spray has dried on the tree (although its protectant cover may be reduced after 50-60mm of rain) and is not affected by temperature fluctuations, so can be applied any time through the season, within the resistance management guidelines of a maximum three sprays per season.

“Recommended optimum use is in a preventative program from pink bud to early fruit development. For an excellent fruit finish, unlike some fungicides, Stroby does not cause fruit russet when used as directed,” Dr Morris said.

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