Some more pests and disease around the botanic gardens

1. Leaf miner

There are a number of different insects whose larvae feed inside the leaves. They can be referred to as "leafminers." Adults may be beetles, wasps, sawflies, moths or flies. They lay their eggs on or in the leaf. Each species always produces the same shape and size "mine." Some look like blisters, some as in this case, look like meandering lines. A wide range of plants are attacked. Control is by removing affected leaves promptly.

2. Case moths (order Lepidoptera)

The larvae of these moths make bag-like structures to protect themselves while they feed on foliage. Different species will have different ways of covering these cases - some have no coverings at all. Below is an example of a case moth larvae which has covered its case with pieces of twig and leaf. It is very well camouflaged. The case increases in size as the larva grows. When fully fed, pupation occurs inside the case. Female moths are wingless and lay eggs inside the case. The larvae that emerge will disperse and make their own cases.

A wide range of plants are affected by different species of case moth. Control is by hand removal of the cases.

3. Leaf Spot - Iris x germanica

Below is an example of this disease which is caused by a fungus which attacks plants in their dormancy. It first shows up as tiny brown spots, each with a watersoaked area around it. As these spots enlarge they turn grey and develop brown to red margins. The spots eventually coalesce to form large dead areas and the leaves are killed. Ongoing spread can weaken plants and hinder flower production.

Worse in warm, wet weather. Fungal spores are spread about by the wind and by water splash. Irises grown from rhizomes commonly have this disease. Irises grown from bulbs are less affected.

Control is by removing affected foliage and destroying. In severe infections, a copper oxychloride spray can be used. A suitable wetting agent can help the spray stick better to leaf surface. It may be necessary to repeat treatment every 10-14 days in ongoing humid, wet weather.

4. White rust - Capsella bursa-pastoris

This fungal disease affects plants in the Brassicaceae family. It is not related to the group of disease generally referred to as rusts. Usually it is characterised by small raised yellowish-green spots on upper surface of leaves with corresponding white soft-looking spore masses on the undersurface. These spores are carried to other plants by the wind or by insects. Badly affected leaves are misshapen. It is most serious in cool, moist conditions.

Control of weeds is important - Capsella bursa-pastoris (shepherd's purse), Coronopus didymus (bittercress), Raphanus raphanistrum (wild radish) are part of the Brassicaceae family. Crop hygiene is important - crop rotation and destroying diseased plant material. Spraying is not effective.

Ref : What Garden Pest or Disease is That? Judy McMaugh

Preparing a site for planting in a larger scale production garden

During this practical session on April 30th, we worked in groups to prepare an area for autumn-winter planting of brassicas for the organic students at TAFE.

The soil in these areas are clay-based, (on feel somewhere between a sandy clay loam to a clay loam) and quite sodic.

Consequently, surfaces collapse easily during rain or irrigation, with the collapsed materials forming a dense surface crust. This impedes infiltration of water into the soil profile and inhibits emergence of seedlings. It can also influence drainage and removal of excess water from the profile as well as limiting the supply of oxygen to subsurface root systems and beneficial microorganisms.

The soil breaks into large clods which make for a very rough seedbed. Ideally, soil aggregates should be between 0.2 - 3mm in diameter.

Improving the structure of such a clay soil involves :

1. the addition of gypsum (Calcium sulphate) given that the soil is dispersive

2.addition of plenty of organic matter

3.judicious cultivation to break up hard and compacted ground (ie. only when soil conditions are right - not too wet and not too dry).

Dolomite (Calcium carbonate/Magnesium Carbonate) was added to raise pH of these areas.

In smaller garden beds, the double digging technique is a good way of mechanically relieving compaction, crusting in the top soil or hardpanning in the subsoil. For larger scale production areas such as the one in TAFE this is not necessarily practical. In this case, a rotary tiller and larger agricultural rotary tiller was used as shown below :

As always, there are benefits and disadvantages of using these machines.

Buried weed seed is brought up to the surface enabling them to germinate. Rotary tillers also can create a compacted layer at the base of the tilled soil. Aggregates in the tilled soil become pulverised. The organic matter that binds aggregates are exposed to attack by microbes. This is why repeated cultivation can steadily destroy a soil's structure. Mechanical tillage like this should only be done once and lightly - once a year probably at most, but with the maintenance of high levels of organic matter.

P.P.E. used during the operation of these machines includes eyewear, gloves, full sleeve shirt and long trousers, steel cap boots, ear muffs.

The smaller rotary tillers are propelled forward by the rotating tines and do not have powered wheels, although they may have small transport control wheels. An adjustable tine acts as a brake, slowing the forward moving machine due to friction as it passes through the deeper untilled soil. The operator can adjust the amount of friction/braking by raising and lowering the handlebars of the tiller. These machine can be pulled backwards or put in reverse to move over areas again, but care should be taken so that the operator does not stumble and pull the machine over himself. Both these machines had gear boxes to allow the forward speed to be adjusted whilst the rotation of the tines stays constant. The larger agricultural tiller has a dead man switch which when released stops the machine from moving forwards or in reverse. Unseen sub-surface objects, such as stones or buried garbage, are also potential hazards as they can abruptly and violently move in any direction.

The benefits of using these machines include efficiency and coverage of a large area, improving drainage and aeration which reduce potential for plant stress and the potential for pathogens.

Reference : Kevin Handreck, Gardening Down Under

Wikipedia - Rotary Tillers

Petal Blight in Rhododendron spp.

The buds and flower is the only part affected by this fungal disease. Humid conditions favour the spread of this fungus, which initially starts as small circular spots that quickly enlarge into irregular blotches.

Affected flowers dry out and turn brown and papery. They remain stuck to the plant for some weeks, unlike healthy flowers which would fall from the bush once faded. Grey mould can look similar and only affects petals.

The specific fungus particularly affects Kurume and Indica azaleas.

Control by collecting and destroying dead and diseased flowers, but spores can blow from plants considerable distance away. Spray with triadimefon as soon as buds show colour in humid weather if severe infection.

Reference : Judy McMaugh, pg 269

Iron Deficiency in Griselinia lucida

This example of iron deficiency was photographed in the RTBG.

When a plant is unable to take up enough iron, leaves become light green fading to pale yellow or even white. This happens gradually and the veins remain green, until the end of the process. Youngest leaves are affected first. In mild cases, the leaf size and new growth appear normal, but if severe, can reduce the size of the leaf and cause dieback. Fruit crop will be reduced.

Handreck suggests a quick and easy way of checking whether yellowing of leaves is due to iron deficiency : spray the plant with an iron chelate. Greening within a week indicates iron deficiency.

Problem often occurs as a result of soil pH being too high. An attempt to lower the pH can be made but this is a slow means of correcting the problem.

Handreck recommends treating the deficiency with the use of an "iron compost" which essentially involves adding iron sulphate to either compost, peatmoss or animal manure in the ratio of one cup (about 230g) to 10L, then digging holes around the rootzone and filling holes with this mix (see Gardening Down Under pg. 146).

Reference : Judy McMaugh pg. 254, Gardening Down Under, Handrek

Target Pruning and Cutting off a branch

Trees recover most rapidly from large cuts if they are smooth and do not damage the branch collar. If the branch is large, remove the bulk of it first to minimize the weight of what will fall last. First, an undercut is made, sawing to about a quarter of the way into the branch from the underside. This partial cut stops the bark tearing down the trunk if the branch accidentally breaks. Second, cut squarely from the top of the branch about 5 cm beyond the undercut (further away from the trunk) until the branch falls away - the cut should be made to the same level as the undercut. The finishing cut can the be made by following the line of the branch collar from top to bottom . Generally, the line of the cut is approximately 90 degrees to the branch bark ridge.

(above) Note first cut is an undercut and second is a top cut a little distance from the first.

(above) the same area, third and final cut must be smooth. Cut just beyond the branch collar at an equal and opposite angle to that made by the bark ridge on the main stem.

Below is a photo of a cut to remove included bark :

This is a weak point of trees and shrubs. Bark is embedded between opposing branches, creating a structurally weak point. It is best to eliminate these branches as soon as they are noted. If not, as the tree enlarges in diameter and weight, the tree is likely to split at this weak point.

In cutting much larger branches that could potentially be problematic in terms of where it will fall, a wedge can first be made to determine the direction of the fall :

Ref : RHS pruning and training manual pg. 12, 22

Scarification trial

Many seeds such as acacias have very hard seed coats and require a physical method to allow water to enter the seed and break dormancy. Such a process is called scarification. This can sometimes be done with sandpaper, using nutcrackers or nicking with a knife depending on the type of seed. Soaking in hot (not boiling water) for up to 48 hours is another method. Seeds must be sewn directly after the soaking period and must not be allowed to dry otherwise they die.

This trial was to compare germination rates of Acacia verticillata and Dianella tasmanica when seeds had been scarified by soaking in hot water for 24 hours compared with seed that had been given no prior treatment.

10 seeds of treated and 10 seeds of non-treated A. verticillata were sewn in different punnets. The same was done for D. tasmanica. This occured on the 23rd April.

Photos of A. verticillata punnets taken on 19th May, about 4 weeks later, with top punnet containing treated seeds and bottom punnet non-treated :

After 6 weeks (non-scarified on left and scarified on right) :

Note the strike rate for scarified seed is 8 out of 10, whilst non-scarified is 2 out of 10.

This shows that germination rates are higher when Acacia verticillata has been scarified.

At 4 and 6 weeks no Dianella tasmanica had germinated, as is expected with the time it normally takes to germinate.

reference : RHS propagating plants pg 53-4

leaf bud cuttings

Most Camellias can be propagated from leaf bud cuttings of current season's growth that has begun to firm (ie. semi-ripe cuttings, where the base of the cutting is quite hard and the tip of the cutting is still actively growing and thus still quite soft). These cuttings include the leaf and axillary bud or node. This allows for many cuttings to be taken from the same stem. Cuttings are made the same size to allow ease of handling by ensuring most cuttings take root at roughly the same time. These are placed into the prop. mix after each cutting has been dabbed in rooting hormone gel, leaving the bud on the surface of the mix. Flower buds are pinched out. With bottom heat, rooting usually takes 6-12 weeks. 

We also took some leaf bud cuttings from a potted Ficus elastica cultivar. Using secateurs, a cut was made straight across a stem just above a node - a rule of thumb is about the distance of the width of the stem itself - and about 2.5cm below the same node. Keeping the waxy side of the leaf outermost, the leaf was rolled to form a cylinder, secured with a rubber band and placed into the prop. mix after a dab in semi-hardwood rooting hormone gel. The lead node should sit on the surface of the mix. The cutting can be supported with a small cane through the rolled leaf as shown above. This is kept in the misting house under humid conditons until rooted. 

Larger leaf Camellia varieties can be handled in a similar manner as shown below : 

Ref : RHS propagating plants, pg. 95, 120