Sustainable disease management and control of strawberry fruit rots - Monika Walter, HortResearch
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Sustainable disease management and control of strawberry
fruit rots – Monika Walter, HortResearch
The aims of this project were to improve current disease control methods, and to
evaluate current and new fungicides, while satisfying pesticide residue and
registration requirements. The influence of soil amendments on plant health and fruit
rots was also investigated.
Laboratory and glasshouse trials were used to screen over 20 biological and
conventional fungicides to pre-select field treatments. Grower trials were conducted
for 3 growing seasons to determine the efficacy, frequency and timing of product
applications. The relative importance of fruit rots under New Zealand commercial
production conditions was determined. This included the importance of latent
infections v. infections resulting from surface contamination at or during harvest.
Key Results (2004-2007)
1. Soil compaction and the lack of aggregate stability were identified as a major
issue for strawberry growers. Soil management recommendations have been
formulated (Annual Report 2).
2. From the 24 products tested in the laboratory using leaf, flower and fruit
assays to determine their control potential for Botrytis cinerea, anthracnose
(Colletotrichum acutatum) and leak (Rhizopus and Mucor spp), 10 chemical
and 2 biological fungicides were selected for further field trials (Annual
Reports 1 and 2).
3. The two commercially biological fungicides selected were found to be
inconsistent in controling B. cinerea in the field in the Auckland and Hamilton
growing regions (Annual Reports 1, 2 and 3).
4. The 10 chemical fungicides tested for strawberry fruit rot control have shown
mixed success. The most consistently performing product was Switch, also
showing some small reduction of leak by delaying the onset of leak rots
(Annual Reports 1, 2, 3, Final Report, plus separate study on Switch and
Pyrus registration).
5. Good B. cinerea control was also achieved with Teldor+Captan Flo applied
as a tank mix, although data is only available for one season (2006).
Pristine also holds promise for B. cinerea control (Annual Report 3, Final
Report). Applications of Pyrus were inconsistent in achieving Botrytis control
in the field (Final Report, plus separate study on Switch and Pyrus
registration).
6. Monitoring of strawberry fruit diseases at commercial grower properties for the
duration of this project and results from our field experiments have
consistently shown that captan (the main fungicide used by growers at 5-10
day intervals) does not significantly reduce any strawberry fruit rots. There
was no effect on control of B. cinerea, anthracnose, leak, other or total rots
observed in any of the field assessments.7. Botrytis cinerea isolates from New Zealand strawberry fields were found to
exhibit a range of sensitivity to captan (Annual Report 3).
8. Strawberry fruit rots were caused by latent infections occurring during
flowering as well as by fruit surface contamination at or during harvest.
Disinfection experiments have reduced post-harvest storage rots. Equally,
monitoring of fruit infections levels during harvest (i.e. in the field and after
packaging) has shown that micro-organisms accumulate on the fruit surface
because of the current handling and harvest processes (Annual Report 3 and
Final Report).
9. Strawberry fruit rots are caused by a complex of pathogens: B. cinerea
dominates rots in the early to mid season, while leak rots dominate the second
half of the production season. Anthracnose, caused by Colletotrichum
acutatum, seems to be of little significance to fruit growers (Annual Reports 1,
2, 3 and Final Report). This is attributed to the relatively cold temperatures
during the fruit production season. Anthracnose favours temperatures well
above 20ºC.
10. Inoculum sources for Botrytis were identified as necrotic leaves and wilting
leaves in the plant and decaying fruit, in the plant or removed and thrown into
the aisle. The inoculum sources for leak is predominantly fruit-to-fruit spread,
resulting from airborne inoculum from actively sporulating fruit in the plant or
aisle, pickers’ or graders’ hands, as well as touching fruit during the various
harvest processes (Annual Report 3, Final Report).
11. In the Auckland growing district, climatic conditions resulted in consistently
high Botrytis disease risk, as validated with Botrytis risk prediction (e.g.
Broome model) (Annual Reports 2, 3 and Final Report).
12. Postharvest fruit rots should be of greater concern to New Zealand strawberry
growers than fruit rots actually expressing in the field. Field rots were
monitored during this last season to prove this point (Final Report).
Throughout all the other field studies we consistently noticed very few field
rots but experienced in the order of 60% postharvest fruit infections.
13. Leak organisms were found to show some tolerance to cold temperatures,
with a third of the isolates tested still growing at 5ºC (Final Report).
14. Postharvest fruit management is paramount, affecting harvest practices and
temperature storage, chilling being paramount for an extended fruit shelf live
(Final Report).
15. Fruit rots doubled as a direct result of harvest handling.
Key Recommendations
1. Results indicate that use of captan by itself as a fungicide for Botrytis cinerea
control in New Zealand strawberry production does not always provide
effective control. A tank-mix application of captan with Teldor for B. cinerea
control, however, warrants further evaluations.
2. The product Pristine also showed promise for B. cinerea control and justifies
further studies.
3. The registration process for Switch should continue to be actively pursued
for use in strawberries for Botrytis control.4. The product Pyrus was inconsistent in B. cinerea control on strawberry and
thus further steps towards registration are not recommended.
5. Removal of inoculum sources, especially infected fruit and leaves, is
recommended and has been shown to reduce disease pressure in overseas
studies.
6. Growers need to improve harvest and postharvest fruit management to reduce
fruit-to-fruit contamination as well as postharvest storage rots. Improved
sanitation and hygiene practices in the field and during picking, grading and
packaging will be important.
7. The importance of leak postharvest rots should not be underestimated by
growers.
8. Potential emergence of cold-tolerant leak isolates and their significance need
to be further explored.
9. Adequate fast and continuous chilling or quick sales are needed for improved
postharvest fruit quality.
10. New Zealand and Australian strawberry plants have a large foliar biomass (in
contrast to plants grown in the USA, for example). Plant spacing, nutrition and
cultivar selection may be useful tools for managing the microclimate, disease
built-up and disease risk.
Cross-contamination of fruit rots during harvest
Strawberry leak is a common storage rot caused by several different species of
Rhizopus and Mucor. Infection occurs entirely through contamination of wounds,
thus requiring an external inoculum source. In order to identify the major points of
contamination by leak organisms in the strawberry production system, samples were
taken from the plant and at several stages through the process of picking, grading
and packaging fruit.
For 9 properties (7 in the Auckland region and 2 in Hamilton) a system using sterile
gauze swabs to test surfaces for contamination by leak diseases and food-borne
diseases was employed. This involved wiping surfaces with a swab that was then
placed into a plastic bag. At each property the hands of 5 pickers and 5 graders were
swabbed as were the surfaces of 5 picking containers and 5 grading benches. Five
fruit were also collected individually into 70 ml specimen containers from plants in the
field and another 5 from chips after packaging. These samples were returned to the
laboratory for processing.
At each property, 5 replicate samples of 10 fruit were harvested from plants in the
field, 5 replicate samples of 10 fruit were taken from the picker’s container, and 5
replicate samples of 10 fruit were sampled from chips after packing. Each fruit
sample was picked into and incubated in a clear plastic egg carton for 4 days at 4ºC
and then for 2 days at 20ºC. Fruit were assessed for the presence of Botrytis, leak
and total rots after 4 and 6 days.
Rhizopus spp. were detected using the washing and plating technique at all
properties surveyed. However, yeasts were highly prevalent (approximately 1000times the level of leak organisms), and therefore leak causing organisms could not
be easily counted. Nonetheless, leak was detected on fruit and on all surfaces
monitored (Figure 1). Both leak and yeast levels followed a similar pattern and were
greater on fruit in the packing tray than on fruit picked off the plant. Organisms were
also readily found at high levels on pickers’ hands, pickers’ containers, the grading
bench or graders’ hands. This suggests that fruit contamination increases during
harvest and the corresponding handling processes. This increase of contamination
was also reflected in an increase of post strawberry fruit harvest rots (Figure 2). All
rots, Botrytis, leak and other rots increased due to handling of the fruit. Total rots
doubled (Figure 2).
10000
Leak organisms
All yeasts (x 1000)
CFU/sampling unit
1000
100
10
1
Field Packed Pickers Pickers Bench Graders
containers hands hands
Fruit Surfaces
Sampling unit
Figure 1: Average number of strawberry leak-causing organisms (Rhizopus and
Mucor spp) and yeasts expressed as colony forming units (CFU) per sampling unit
(i.e. one fruit, picking container, hands or 15 × 15 cm bench area). Nine packhouses
were surveyed.6
Plant
5 Pickers tray
Packaged
4
Fruit rot (%)
3
2
1
0
Botrytis Leak Total
Causal organism
Figure 2: Increase in strawberry fruit rots caused by Botrytis cinerea and leak-
causing organisms (Rhizopus and Mucor spp) during harvesting. Fruit was collected
directly from the plant, pickers’ trays and packaged punnet. Fruit were incubated for 4
days at 4ºC followed by 2 days at 20ºC. Nine growers were surveyed.
Funding for the different projects was received from Strawberry Growers New
Zealand Inc., Ministry of Agriculture and Forestry Sustaiable Farming Fund (MAF-
SFF Grant #03/197) and Syngenta Crop Protection Ltd. In-kind support has been
received generously from numerous growers, consultants, New Zealand and US-
based scientists and researchers. Thanks also to all participating chemical and
biological fungicide producers and suppliers.You can also read
