Lessons from the past installment 3: Leaf spot

We’ve been reviewing the materials we have on the industry spray list for leaf spot.

While Megastar (flusilazole) doesn’t tend to show up in fruit residues, it can appear in powdered products (concentrated 60-100x).  This is problematic because it is not registered anymore in Europe, Japan, or in most of our blackcurrant markets, effectively making it a banned product.  Topas (penconazole) is currently registered in most currant export markets (though not in the USA).

As part of our review, we looked at the use pattern for our two leaf-spot fungicides, and compared that to the life cycle of the fungus.  Fungal life cycles are always interesting, and in this case relevant to choosing an effective application timing.  Below are the most important bits, from a scintillating 1989 article titled “Mycosphaerella ribis leaf spot on black currants in New Zealand: Perithecial maturation, ascospore release, and symptom development,” by Robert Beresford and Ron Mulholland out of the DSIR days.

In NZ, leaf spot on blackcurrants is caused by a fungus called Mycosphaerella ribis, which is different from the fungi that cause leaf spots on currants in Europe and the USA.  Its asexual stage is known as septoria (Septoria ribis).

This fungus overwinters on fallen blackcurrant leaves that had leaf spots the previous summer.  Over winter, the fungus grows its sexual structures, microscopic pustule-like structures that release spores in the spring.  The fungus is clever and times its spore release with the appearance of new blackcurrant leaves.  New infections take 2 weeks to become visible leaf spots, and another 3 weeks to grow new spore-forming structures.  That means that an infection that happened in late September would probably become apparent in the second week of October.

Spore release from the overwintering leaves is finished around grape stage of blackcurrant development.  That means that all subsequent leaf spot infections happen within the canopy from the asexual spore formation on in-season (Septoria phase) leaf spots.

The important bit to note is that if the spring leaf infection stage can be prevented, then for the rest of the summer, leaf spot is not an issue.  It can only infect later in summer from current-year’s leaf spots, as the spores from last year’s fallen leaves are all spent.

The time to prevent leaf spot is from green tip to grape stage. Fortunately, the primary infection stage finishes before flower opening, meaning it should be possible to keep fungicide residues out of fruit.

The spray diaries show that many growers are applying Topas to control secondary infections and missing the more important earlier time frame.  Variety resistance is also an important consideration when choosing a spray strategy.  For Blackadder, Murchison and Magnus, uncontrolled leaf spot can cause severe leaf lesions and early defoliation, reducing the yield potential for the following year.  But  other varieties like Ben Ard are much more tolerant, and it is reasonable to question the amount of spraying that should be done for leaf spot control.  Ben Rua, Lewis, Kepler are intermediate.  Our present thinking is to have 3 spraying regimes.

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