Rind texture is mainly influenced by two factors: crop load and nutrition. Crop load has the greatest influence on rind texture, however, nutrition can help to alleviate and/or offset some of the crop load's effects.
Rind texture is coarsest when a low crop load occurs. Trees obtain their energy from the sun and photosynthesise it into carbohydrate. During a low crop-load year, the tree is making more carbohydrates than is required to grow the crop.
Some of this extra carbohydrate is stored in the roots and trunk of the tree, however, some of it is also put into the fruit, namely the rind. The rind becomes thicker and coarser, most prominently around the neck of the fruit. There is a positive effect of low crop load: larger fruit size, but in some instances it can also grow too large and be unmarketable.
The simple solution is to produce a balanced crop load every year, which is easier said than done! However, you can manage some of the variability in crop load. The best strategy is to grow a balanced tree that is more inclined to bear even crops.
Firstly, apply the basic principles of irrigation and nutrition to ensure trees are not stressed. A stressed tree can sometimes over flower and either set a heavy or low crop load. Secondly, monitoring each of your block's crop load is important to help identify trends and any need for regulation.
A balanced tree has enough shoot growth over the spring and summer season to bear flowers for next season. Too much shoot growth can result in an excessive crop and too little could produce a low crop. Pruning is an excellent strategy to help keep trees in balance.
Pruning helps to promote new healthy shoots that bear strong healthy flowers (leafy inflorescence) and good quality fruit. Pruning is best done in winter to improve flowering in the current season. If the tree has too many shoots then light hedging could trim off excessive shoot growth. An early indicator for the next season is the amount of shoot growth during spring and summer. Monitoring at this time allows adjustments to be made (i.e. nutrition management or hedging/pruning).
Pre-emptive crop regulation is manipulating the tree in winter to help it set a good crop load and fruit size right from the start, at flowering. Ralex® is a registered form of gibberellic acid (GA) that is sprayed onto trees in early winter not only to reduce flowering, but also reduce the number of leafless inflorescence that has a low fruit set and smaller fruit size.
Winter urea sprays have been trialled overseas to increase flowering and crop load, however, it has not been properly trialled in Australia. Research conducted at the NSW DPI Dareton site could not find a response to winter urea application on young, mid-season navel trees, however, a response might occur in older trees in a different situation (i.e. soil type; rootstock etc.).
Post-flowering crop regulation is what you do when trying to correct a problem that has already occurred. It is generally used when an excessive crop is set.
Options are chemical thinning, hand thinning and pruning, or hedging. Crop thinning reduces stress on the tree so it can set a higher crop load on the following season, although, in some situations, there could be some increases in fruit size in the current season.
Chemical thinning is complex as it is done in October through to early November when there is a risk of further fruit drop after treatment. If a block has a consistent pattern of biennial bearing and evidence of a current excessive crop load, chemical thinning is a viable option.
Local and overseas studies have indicated that GA applied during flowering can also help to increase fruit set in some situations. There was no response at a trial conducted by NSW DPI on a grower's property in Curwlaa, NSW for Chislett navel oranges when sprayed at full bloom and 90% petal fall.
However, a response was detected in a NSW DPI trial conducted in the Murrumbidgee Irrigation Area (MIA) on the Midnight common orange. GA applied at flowering is recognised for its variable results and, apart from seasonal effects, it is probably more effective on some varieties than others.
Research has demonstrated that nitrogen significantly contributes to rind texture. Over the years, nitrogen application has been increasing to help improve the vigour and productivity of trees. Trees might be looking dark green, healthy and vigorous, but the downside is that rind texture becomes coarser.
The simple solution is to reduce nitrogen application to a point where trees have adequate nitrogen and remain productive, but have smoother rinds. There is no 'one size fits all' solution – each block needs to be assessed separately. It's important to make gradual changes and annual leaf analysis is an excellent tool to manage and monitor changes over time.
Excessive potassium can also increase rind coarseness, but has a lesser effect on rind coarseness than nitrogen. Due to cost, most growers do not apply excessive amounts of potassium.
Phosphorus is also important because it helps to counteract the effects of excessive nitrogen. However, once adequate phosphorus levels are reached adding more phosphorus has little effect. Some growers have been using mono ammonium phosphate sprays (MAP) as a way to quickly boost tree phosphorous levels. Little is known about this spray.
Fruit has a high demand for nutrition and the nutritional demands of a tree would depend upon crop load. In a high cropping year, more nutrition would be required and vice versa. Table 1 provides a summary of average nutrient crop removal rates of oranges (Paramasivam et.al. 2000).
This can provide an insight into the changing nutritional requirements of trees at different crop loads. By the middle of December, most growers can make a reasonable assessment of their crop load and use this table to guide them on how much to increase or decrease their level of nutrition.
A greater understanding on practices to produce smoother rinds in Australian conditions and citrus nutrition can be obtained by participating in the NSW DPI Navel smooth rind project. Growers can also participate in the pilot Citrus Diploma coordinated by SuniTAFE in Mildura.
Paramasivam S, Alva AK, Hostler KH, Easterwood GW, & Southwell JS (2000). Fruit nutrient accumulation of four orange varieties during fruit development. Journal of Plant Nutrition, 23(3), pp.313–327.