"Organic" Alternatives to Black Spot and Powdery Mildew Fungicide Sprays
Before the development of synthetic fungicides in the 1940s, a wide range of non-synthetic materials were used to control fungal diseases on pipfruit. These pre-modern fungicides were not as “organic” as you may think - some compounds such as phenyl mercury and naphthoquinone compounds are banned today because they do not meet modern public health requirements. However two non-synthetic fungicides, sulphur and copper, are acceptable organic sprays (with some restrictions) and have been reinvestigated by HortResearch for use on modern orchards. Other compounds which can be classified as organic, including slaked lime, have also been evaluated by HortResearch scientists.
The Best "Organic" Alternatives
Table 1 summarises the main results. These results are amalgamated from trials undertaken by Dr Beresford and others at Winchmore Orchard (Canterbury) from 1989-1995 and at Clyde and Havelock North from 1993-94. The treatments are rated according to relevant performance and are not compared with synthetic fungicides. (See the section below “More Details of the Trials”)
Table 1:
| Treatment | Black spot control | Powdery mildew control | Degree of russeting | Other effects |
| Cupric hydroxide | Very good | Very good | Severe | |
| Copper sulphate and sulphur | Moderate | Moderate | Moderate | Highest fruit yield |
| Bentonite clay | Moderate | Poor | High | Lower fruit yields |
| Vegetable oil | Poor | Good | High | Lower fruit yields |
| Slaked lime | Good | Poor | Low | Chalky deposit on apples |
| Slaked lime + Cupric hydroxide | Good | Poor | Moderate | Chalky deposit on apples |
| Cupric hydroxide + sulphur | Very good | Good | Moderate | |
In additional trials, some of the compounds were alternated with a sulphur spray. The alternate sulphur sprays improved the black spot control of cupric hydroxide, copper sulphate and vegetable oil but not bentonite clay.
Over all, cupric hydroxide gave the best black spot and powdery mildew control, but also resulted in high levels of russet. When applied at rates low enough to avoid russet (below about 50g Cu/ 100 litres), fungal control was poor. In Clyde, no copper was applied over the russet sensitive period (mid October to early November in this case), but russeting still occurred.
Cupric hydroxide mixed with sulphur gave good disease control but the fruit still showed russet. This is thought to be due to the copper in the mixture.
Therefore, if fruit finish is an important consideration, slaked lime may be a better alternative. Slaked lime gave poor control of powdery mildew even when used in conjunction with cupric hydroxide, so alternatives to copper are still required for powdery mildew during the russet-sensitive time (during and up to one month after flowering).
Slaked lime did not appear to affect fruit colour although it did produce a white chalky deposit on the apple surface. This deposit could potentially be wiped off at harvest or removed in the water dump at the packhouse. Slaked lime degrades to calcium carbonate in the air and is considered environmentally safe.
Costs involved
Slaked lime used in the trials cost $0.22/kg. At the most effective rate tested (2.2kg/100 litres) this would cost $0.48/100 litres applied. At an application rate of 2000 litres/ha the cost per hectare works out at $9.68/ha.
The cupric hydroxide formulation used cost $8.70/kg. If the optimum application rate to minimise russet and maximise disease control was 32g of formulated product/100 litres (16g copper/100 litre), the cost would be $0.28/100 litres applied or $5.56/ha at 2000 litres/ha.
Both these compounds are relatively in-expensive compared to a standard synthetic fungicides.
More details of the trials
The first set of trials (Beresford et al, 1991) were conducted on three year old Royal Gala trees at the Winchmore Research Station in Canterbury during the 1990-91 growing season.
The compounds used were :
Each treatment was applied five times during the primary black spot infection period between 10 October (early pink) and 12 December 1990. In addition, a further set of treatments alternated a sulphur spray with each of the compounds to give ten fungicide applications during the primary infection season. After the 12 December, all plots received a standard programme of fortnightly cupric hydroxide and sulphur until harvest.
On the 6 December, the levels of black spot and powdery mildew were assessed on the leaves, and black spot and russet were assessed on the fruit.
The second set of trials, (Beresford et al, 1995) were carried out at Winchmore, and also at Clyde and Havelock North Research Orchards.
The trials at Winchmore were to determine the effects of different concentrations of cupric hydroxide and slaked lime. Shell Kocide 101 (50% Cu) was applied at 12.5, 25, 50, 75 and 125g Cu/100 litres. The four application rates of slaked lime used were 0.4, 0.8, 1.6 and 2.2 kg/100 litres of water. The sprayer pump could not handle concentrations of slaked lime much more than 2.2 kg/100 litres.
The trials at Havelock North and Clyde compared the following treatments during the 1993-94 season:
The cupric hydroxide formulation was Shell Kocide 101(50% Cu) and the sulphur was BASF Kumulus DF (80% sulphur).
Black spot, powdery mildew, russet and fruit colour were all measured at harvest according to export grade standards.
Future Work
Black spot and powdery mildew on pipfruit are very difficult to control under an organic or reduced-spray programme. Some organic fungicides have shown to give good control without compromising fruit quality. The next stage is to develop an organic spray programme which uses combinations of the organic compounds which will control black spot and powdery mildew without causing russet.
Acknowledgment
HortResearch acknowledges the Foundation for Research, Science and Technology for their support.
References
Beresford. R.M, P.A.G. Elmer, M. Spink, R.T Alexander And M.J. Daly (1991) Fungicides for Control of Black Spot and Powdery Mildew in Organic Apple Systems. Proc. 44th N.Z. Weed and Pest Control Conf. 1991 : 86-90.
Beresford. R.M, C.H. Wearing, J.T.S Walker, M. Spink, R.R. Marshall And V. White (1995). Copper and Slaked Lime for the Control of Black Spot and Powdery Mildew in Apples. Proc. 48th N.Z. Plant Protection Conf. 1995 : 83-88.