June99

A significant number of fruit trees in a large number of orchards are showing symptoms of crown rot due to "wet feet". Infected trees are unthrifty, exhibit poor extension growth, and are stunted.

Foliage is sparse and chlorotic and may have developed an early purple discoloration in autumn (1998). Fruit tends to be small and colour red/yellow prematurely. Trees usually decline progressively over several seasons and eventually die. However, trees are collapsing and dying suddenly now following an extremely wet year when the water table remained high.

The roots of the infected tree are dying from the bottom up. I have found these symptoms in orchards ranging in age from 30 years old to orchards planted in spring 1999. All rootstocks are showing symptoms of crown rot.

Most damage is already done by the time foliar symptoms betray the presence of the disease. The following recommendations may encourage new growth in affected trees:

Site selection and soil water management is most important in preventing crown rot. Apple trees should be planted only on soils with adequate drainage throughout the year. Internal drainage should be improved by subsoiling (short-term advantage), shoring (long-term advantage) and/or planting on ridges. Mole draining is beneficial in the short-term and can be carried out with success if ground conditions are dry in the summer.

Affected trees with no leaf now will not survive.

Trees with any leaf area healthy have a chance of surviving. These trees should be prunned back to half leaving the healthiest limbs alone. This should give the few remaining healthy roots less to feed.

Apply 10 cms (4 inches) deep mulch of farmyard manure, under the tree canopy. The purpose of the mulch is to retain moisture at/near the soil level where the healthy roots are. The mulch will also encourage new feeding roots to be formed in the top 15 cms (6 inches) of soil.

Spray young orchards for crown rot protection after blossom and repeat after 3 - 6 weeks with Aliette, a systemic phosphonic acid fungicide.

Ensure that all trees in all orchards are properly tied to the stake.

With a combination of the above recommendations, growers may see an improvement in tree performance towards the end of the year.

Growers must follow the manufacturers' instructions on the product label

As a result of the recent rain, the scab risk remains high. In previously clean orchards, the scab innoculum pressure is high from nearby sources.

Heavy rain will reduce the effective protective period of the fungicide.

Shortening your spray interval will reduce the risk of an infection in your orchard.

When growth is fast, new leaves develop quickly. These new leaves remain unprotected until the next spray.

The following table gives a comparison of apple scab infection periods recorded at NIHPBS, Loughgall from 1 March to 9June for the 1998 and 1999 period:

Comparing the number and length of scab infection periods recorded in 1998 with this year, the 1998 season would initially appear more severe. A total of 21 infection days were recorded in the first three months between March and May in 1998 compared to only twelve over the same period in 1999. However, comparison of the length of these infection periods, i.e. infections lasting one or more days, reveals an identical number of eight periods in both years.

The greater number of infection days in 1998 can be attributed to a very wet April. Almost 90 mm of rain were recorded in 1998 compared to 64 mm in the same month this year. The first half of April 1998 was particularly wet with a continuous infection period of 8 days recorded between the 2.4.98 - 9.4.98. This is considerably greater than the longest infection period recorded for this year, which lasted only 3 days from the 11.5.99 - 13.5.99.

Any infection period that lasts 8 consecutive days poses a particular problem to the grower. Most 10 day spray programmes are likely to run-out during an extended infection period and finding a dry day/period in which to apply the next spray poses a particular problem. So far, the infections in 1999 have been reasonably well spread ensuring adequate time to apply protective sprays.

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