For gardeners on the east coast – don’t forget that it is a legal requirement in some areas to remove galls from citrus trees by the end of August. Gall wasps lay eggs in main stems or fruit stalks. This forms a swelling and gall wasp larvae weaken trees. As trees may be forming blossoms, delay in pruning may reduce next year’s crop. If you haven’t done so, check citrus trees (especially grapefruit and lemons) this week and prune off any galls before adult wasps emerge to lay eggs in new shoots. Burn affected prunings or dispose of them in a sealed plastic bag. See: Citrus gall wasp
Column 8 in today’s Sydney Morning Herald stated that Sydneysiders’ kitchens have been infested by fruit flies, stating that, “They emerge from fruit and hang around all summer”. The flies referred to are not fruit flies, they are the very small vinegar or ferment flies (Drosophila). Genuine fruit flies are kept out by fly screens. Vinegar flies emerge from fruit, tomatoes, etc. as grubs (larvae) and require pupation outside the fruit in order to complete their life cycle as a fly. If these tiny pests are a persistent problem, there must be a breeding ground nearby or the kitchen needs more regular cleaning.
Vinegar flies are attracted to the smell of yeast in fermenting organic materials and drains. To eliminate the problem:
- Do not keep fruit at room temperature in warm, humid weather
- Cover compost and garbage containers
- Regularly rinse out garbage containers
- Rinse beer and wine containers before recycling
- and treat drains with an enzyme product to break down thick scum where they can feed and reproduce.
Genuine fruit flies, the Mediterranean Fruit Fly (Ceratitis capitata) and Queensland Fruit Fly (Dacus tryoni) cause a lot of destruction in gardens. They require pupation in soil after the maggots emerge from fruit. To reduce the problem of genuine fruit flies, collect all fallen fruit, put it in a sealed black garbage bag and leave the bag in the sun for three or four days to cook the larvae (and encourage your neighbours to do the same). Never put infected fruit in the compost container.
A reader wanted to know what what is causing yellowing of new leaves in her potted, dwarf lemon tree. From the photos she e-mailed, it does look like this tree has an iron deficiency, as yellowing is showing in the young leaves. This can be caused by a number of conditions:
a) potting mix (or soil) that is too alkaline from excess bio-char or calcium in the mix or fertiliser containing a lot of poultry manure
b) cold and wet soil or growing mix
c) if there is a build up of fertiliser salts from synthetic fertilisers, or
d) where there is an excess of potassium from synthetic fertilisers or over-use of seaweed liquid fertiliser.
The first thing to do is check that your pot has ample drainage. Large pots should not sit directly on a hard surface. While smaller pots usually have ample drainage holes around the sides at the base of the pots, large pots often have only one large hole in the base and this can easily become blocked resulting in poor aeration and/or a concentration of fertiliser salts if synthetic fertilisers have been used. Large pots should have pieces of tile placed under the pot to allow a small space between the base of the pot and the verandah or paving. If you notice crusting around the top of the soil line (fertiliser salts), flush the plant with clean water, once drainage has been improved.
The next step is to check the pH of the mix with a test kit. A suitable pH is important to all parts of your garden as the pH in soil or mix controls the availability of nutrients. Test kits are very economical to use and readily available from larger nurseries. If you find that the pH is above 7.2, you could repot the tree using an organic-registered potting mix as organic matter is an important source of iron. However, to do so may result in the loss of this crop of fruit.
The addition of flowers of sulphur (elemental sulphur) is the usual way to reduce pH in soils, but it is easy to overdo this in potted plants. You can apply iron chelates (the form of iron in organic compost) to the mix in the pot at the recommended rate. Citrus trees do not absorb iron chelates well through foliar spraying. Or, you can fertilise the tree with a weak solution of Multicrop’s Ecofish. This is an organic-registered liquid fertiliser that contains soluble iron and has a low pH, qhich will help to reduce the pH in the pot. Ecofish contains iron, manganese, sulphur and zinc (trace elements needed by citrus). Manganese deficiency is also caused by high pH or poorly-drained soil.
I have revised this post to provide more detail because there have been a few enquiries recently about fertiliser requirements for citrus, particularly trace elements.
Time to fertilise
There is no set time of the year for fertilising citrus as different species produce fruit in different seasons. Many citrus trees are producing crops at this time of year (through winter) and it is not a good time to give them a good dose of fertiliser. As citrus go through a growth cycle after fruit has matured, a good general rule is to apply fertiliser to the soil surface, under the outer part of the canopy, after fruit has been harvested, then cover the fertiliser with about 5-8 cm of organic mulch. Avoid scratching fertiliser into the soil surface as citrus roots lie close to the soil surface, and never apply fertiliser to dry soil, as it will burn tree roots. Repeat the fertiliser application in approximately six months, but avoid applying fertiliser in very hot weather.
Suitable citrus fertilisers
Citrus trees require a good supply of fertiliser that contains a full range of both major and trace elements for good growth, and pest and disease resistance. Trees to five years of age can use up to 500 g of complete organic fertiliser per year, as the inclusion of organic matter in soil will make nutrients more readily available. This should be divided into 2 applications. Older trees may require a little more. Very young trees should receive a proportional amount, as they will also benefit from the occasional application of manure tea. Too much high nitrogen fertiliser will attract aphids, scale, the citrus butterflies and citrus leaf miner. Over fertilising can also kill citrus trees.
Compost is the best fertiliser for citrus, but worm castings, poultry-based fertilisers, and well-rotted manures will also keep trees healthy.
Major nutrient elements are: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Part of the role of phosphorus in plants is to promote root formation and early growth. Heavy applications of high nitrogen fertilisers can make phosphorus unavailable to plants. Phosphorus is also only available within a narrow pH range. Keeping the soil pH around citrus trees close to neutral will improve their growth, and phosphorus from organic sources is more readily available to plants. Citrus have a fairly high requirement for the major element magnesium. Signs of magnesium deficiency appear first in older leaves where yellowing begins at the outer edges of the leaves and moves inwards, resulting a green V shape at the stalk end. It can also cause cupping of leaves and lack of sweetness in ripe fruit. This problem is common in citrus in autumn when fruit it maturing. This deficiency can be corrected quickly by watering in some Epsom salts: about 250 g for a young tree up to 2 kg for a fully-grown tree. If pale leaves have occurred on your citrus trees in the past, in future, apply one handful of dolomite per square metre of tree canopy when fertilising after harvest. In soils of SE Queensland that contain high quantities of magnesium, this problem will only occur where far too much potassium has been applied, or where soil has become quite acidic.
Yellow leaves in late winter, or early spring, are often caused by cold soils, if the tree has been adequately watered and fertilised. This problem will correct itself as the soil warms, and the tree begins to extract nitrogen from soil.
As mentioned above, citrus trees require a full range of trace elements. These are: iron, copper, zinc, molybdenum, manganese and boron. Their availability to plants is dependent on soil pH and the presence of organic matter in soil.
Copper deficiency will cause fruit drop and can, in more serious cases, cause gum to form inside and outside fruit, and on shoots. Iron deficiency reduces citrus crop size and causes leaves to gradually become pale green, and then fade to pale yellow, preventing the tree from manufacturing carbohydrates. Zinc deficiency reduces fruit bud formation, and manganese deficiency prevents vitamins forming in fruit. Zinc and manganese deficiency both produce yellow mottling between veins on young leaves, but in zinc deficiency leaves are smaller than normal and bunch together. Boron is essential for flower production and fruit quality, but boron toxicity can be a problem where laundry grey water containing borax has been used for irrigation.
Trace element deficiencies can occur if your soil, or the fertiliser you have been using, does not contain a particular micronutrient. However, they are most commonly caused by your soil being too acid or alkaline where nutrients are locked into compounds that plants can’t absorb. Humus in soil has a pH of around 6.5 where all nutrients are available to plants. Humus is also able to hold trace elements in a form that is easily absorbed and prevent nutrients leaching away through soil.
It is unwise to apply trace elements individually because, as the name implies, they are only required in tiny amounts, and excess applications can be toxic to plants, causing another range of problems.
To avoid trace element deficiencies, add a moderate amount of compost to the soil surface around your citrus trees. Organic mulch will also produce humus after friendly microorganisms break it down, and earthworms distribute it through soil. Also apply a liquid seaweed extract, at weak black tea strength, in autumn and spring to ensure your trees have access to a full range of trace elements. Seaweed also contains a good quantity of potassium to improve fruit quality and build plants’ resistance to pests, disease, frost and drought by strengthening cell walls. If you are experiencing serious problems with the health of your citrus trees, I suggest you test and correct your soil pH, or problems will continue.
Reasonably priced Soil pH test kits are available from most nurseries.