Post-harvest Clean Up Sprays

Leaf Scar Invaders

In apples, and to a lesser extent pears, European Canker (Nectria Galligena) is the only significant disease to infect through leaf scar tissue over the leaf fall period.

With some pear varieties, notably Winter Nellis, and occasionally in apples, leaf scars can be invaded by the bacterial disease Blast (Pseudomonas syringae). Instances of infection of these fruits, with the exception of Winter Nellis, by this disease are rare in New Zealand. In the case of Winter Nellis, it is my opinion that symptoms often attributed to Blast the Pseudomonas infection is probably secondary to either lack of pollination the following spring, or boron deficiency.

In contrast to pipfruit, summerfruit carry a high risk of leaf scar tissue invasion by bacterial diseases. Stonefruit Blast caused by Pseudomonas syringae is by far the most common bacterial spot, caused by Xanthomonas campestris pruni is also thought to be able to enter the plant through leaf scar tissue.

European Canker

Once established, European Canker is by far the most damaging disease found in apple orchards. Fortunately, it is not established in Hawke's Bay, Wairarapa and drier parts of the South Island. It has been established in Auckland, Waikato, Bay of Plenty and Taranaki for many years and has now become established in Nelson and one or two orchards in Gisborne. Its spread out of the Auckland area has been through movement of infected nursery trees or graftwood.

The disease is active in wet or foggy weather and when these conditions occur over the leaf fall period in orchards with established cankers, severe leaf scar infection can occur. The disease has a long incubation period and infected leaf scars and the resultant bud injury is not normally seen until the following spring. The disease is also able to infect winter pruning wounds.

Traditionally pre-leaf fall and leaf fall copper sprays have been used for control of leaf scar infection. Persistent copper formulations such as copper oxychloride and Bordeaux mixture or its substitutes, eg, Cuprofix have label claims for its control. Recent trial work, however, casts doubt on the effectiveness of copper sprays for its control and this work shows that Carbendazim or Euparen Multi are much more effective. It may be a question of Copper formulation or rate rather than ineffectiveness of copper, because in the past there has been very convincing experimental evidence from published scientific papers to show that leaf fall Bordeaux sprays are highly effective.

The new information suggests that a single spray of carbendazim applied before significant leaf fall and wet weather occurs will give better control than the copper programme. The same trials show that two sprays of Euparen Multi, one at the beginning of leaf fall, and one later in leaf fall will give similar control to the carbendazim programme.

While both these fungicides have label claims for use on apples for other diseases, notably black spot, neither has a label claim for European Canker control.

Fungicide resistance is an important issue with carbendazim so its use needs to be integrated into a resistance management programme. With a disease like European Canker, my suggestion would be to only consider using carbendazim every second year and apply the alternative product in the other year.

Suggested rates are:

• Euparen Multi 2 sprays at 250 g/100 l
• Carbendazim 1 or 2 sprays at 60 ml or 60 g/100 l of 50% active ingredient product.

Apply as dilute, high volume sprays.

Bacterial Disease Control in Summerfruit

Pre-leaf fall and leaf fall copper sprays are your only line of defence here.

The aim is to protect exposed leaf scar tissue, so spray persistence is important. Bordeaux mixture and its proprietary substitutes are the best options, followed by coppery oxychloride. As summerfruit foliage is sensitive to copper injury, applications while the leaves are still active need to be kept low, eg, 400 g:600 g:100 l of Bordeaux mixture. In districts subject to frosts before the leaves shut down this early spray is considered very important. Once the leaves shut down, the rate needs to be lifted to a minimum of 200 g metallic copper per 100 l. Where bacterial disease is a problem, a programme of three copper sprays is recommended: one immediately prior to commencement of leaf fall, one when leaf fall gets underway, and one at the end of leaf fall.

Black Spot

Application of 5% Urea, ie, 5 kg/100 l, at leaf fall to apples will reduce Black Spot carry over to next season on fallen leaves. This spray has a direct effect by stopping ascospore development in the spring, and an indirect effect of hastening leaf decay over the winter.

Note: Leaf fall urea sprays are not an effective control measure for pear black spot. This is because pear black spot is capable of carrying over on wood lesions in the tree.

Fireblight

Leaf fall copper sprays are not of any value for the control of this disease.