Why do fruit rots develop?
(continued from last month)
Nutrition
Nitrogen, potassium and calcium are the minerals most often associated with disease severity.
Excess nitrogen results in softer fruit (more easily penetrated by germinating spores) and excessive tree vigour (high humidity within the tree canopy).
High potassium is often associated with increased resistance to disease.
Adequate calcium is essential for strong cell walls, the first barrier to fungal attack.
Persistent rain and/or overcast weather reduces soil water uptake by the tree. Calcium in particular is affected as it is passively taken up in the transpiration stream.
If trees are not taking up water they are not taking up essential nutrients. This phenomenon is often seen in green house hydroponic tomatoes where transient magnesium deficiency occurs during overcast days and disappears on sunny days.
Chemical control program
The following factors need to be considered:
- Use chemicals appropriate to the disease being controlled. Many chemicals used in a Brown Rot control program are not effective against Grey Mould
- Timing to coincide with critical infection periods and fruit development stages
- Flowering period for Grey Mould
- Fruit drop period for Brown Rot
- Fruit cluster closure
- Spray coverage and frequency of spraying. The chemicals applied are as effective as the cover achieved. Ensure that spray volumes, travel speeds and fan speeds are giving full cover of fruits. This is especially relevant when applying protectant fungicides.
Cherry leaves are large and the canopy dense, so spray volumes that may be adequate for apples may not be sufficient for cherries.
How do rots spread?
- Spore dispersal via wind and rain splashing.
- Insects (LBAM, earwigs, carpophilus beetles, etc) transport spores between fruits and clusters.
Critical control points
- Identify the fungal species responsible for fruit rots
- Reduce spore load (inoculums)
- Minimise latent flower and fruit infections
- Limit fungal regrowth from latent infections
- Limit spread when symptoms occur.
These are discussed here.
Identify the fungal species involved
This is critical for developing an appropriate control strategy and an effective chemical control program.
Reducing spore load
Remove infected fruit from trees. This could be combined with post harvest pruning. Infected dry fruits within the canopy are an immediate source of infection for the following season. They are in close proximity to flowers and developing fruit and are difficult to wet with fungicides and sanitisers.
Pay particular attention to rotten fruits that become stuck to branches/spurs and in limb crotches (Figure 1 - see Tree Fruit December 2011 ).
Reduce inoculums on orchard floor. New Zealand research with kiwifruit showed that significant reductions in over wintering inoculums on the orchard floor reduced the incidence of fruit rots.
Mulches formed a barrier to the release of spores from infected material and aided in decomposition of plant debris. Alternatives could be application of 2.5 cm compost or application of compost teas to aid in microbial decomposition.
Post harvest spray program. Many chemicals could be considered including: sulphur, quaternary ammonium compounds, hydrogen peroxide.
Minimise latent flower & fruit infections
- Apply appropriate chemicals throughout the flowering period, especially to minimise Grey Mould latent infections. If fruit rots were a problem the previous season, commence flower sprays at ‘popcorn’.
- Apply a thorough spray program during fruit drop.
- Increase spraying frequency during wet weather or if overnight dews, fogs or mists are frequent.
- Reduce excessive vigour (nitrogen and water inputs), remove water shoots and excessive lateral growth).
- Ensure that post harvest pruning addresses excessive limb numbers (to reduce humidity within the canopy) and tip prune 1-year old laterals on varieties that fruit in clusters.
- Consider using flower thinners or thinning clusters by hand to reduce tight clusters.
- Control insects that damage fruit.
Limit fungal regrowth from latent infections
The two critical factors are, excessive soil moisture and high humidity.
- Prevent excessive soil moisture (good soil drainage, irrigation scheduling and orchard floor management ).
- Low humidity in the fruit zone (canopy management, prevent water pooling in wheel ruts).
Limit spread when symptoms occur
- Reduce compactness of fruit clusters (blossom thinning, fruit thinning, pruning).
- Control insect pests that can spread spores from infected fruits.
- Effective chemical usage (chemicals appropriate to the disease, thorough spray coverage, volume, travel speed, increase spray frequency prior to and during adverse weather events).
Chemicals to control fruit rots
A number of chemicals are registered for controlling Brown Rot in cherries in Australia. Not all of these chemicals are effective against Grey Mould.
Most registered chemicals have a protectant action and only one chemical, propioconizole (Tilt) is fully systemic with eradicant capabilities. The only other systemic group (benomyl) is no longer registered for cherries. Iprodione (Rovral) is partially systemic (translamina).
Compared with some other cherry producing countries (USA, Canada, NZ) Australia has few chemicals with eradicant action for control of Brown Rot and Grey Mould. The loss of benomyl and the restricted use of iprodione (resistance management) makes control of these diseases especially difficult in wet seasons. There is a need for additional chemistry to be registered, especially for Grey Mould control.
Table 1 lists chemicals commonly used by the wine grape industry for controlling Grey Mould (see Tree Fruit December 2011).
The future of bio-controls for controlling fruit rots
The following bio-controls have given positive results against Grey Mould and Brown Rot: (see Tree Fruit December 2011).
FOR the full article with tables and images, see Tree Fruit December 2011
