Magnesium deficiency - azaleas

Mg deficiency first shows up on older leaves. Leaves begin to yellow near the midrib and the yellowness enlarges until the only green remaining is near the tip and near the base of the leaf, giving it a characteristic V shape. Mg is a constituent of chlorophyll and the plant transfers Mg from old leaves to new leaves - thus new leaves remain green in mild cases. Mg is also moved to developing fruit, but leaf fall and crop size reduction can result in severe Mg deficiency. Problem is more likely on acidic soils, but can occur on alkaline soils as well. Can be corrected by spreading epsom salts (Mg SO4) around under plant and watering in or by dissolving in water and spraying onto foliage. 

Infectious variegation - camellia

Probably a virus that causes variable discolouration commonly on camellia spp. - small yellow marks here and there on leaves, groups of yellow marks or yellow leaf margins. It is uncommon for more than a few leaves to show this abnormality. The plant grows and flowers normally. Remove leaves showing symptoms if they are bothering, but symptoms will develop on other leaves from time to time. 

Pestalotiopsis - Gordonia axillaris

This fungus commonly affects camellia spp. and invades areas of leaf that are already damaged or sunburnt. The area becomes silvery grey and may have black pin-prick sized fruiting bodies. Adjust cultural practises to minimise leaf damage. 

Oedema - camellia

Oedema is the term used for the corky brown or grey scabby areas on the leaves. The affected tissue os hard and may occur on only a few leaves. Most frequently on Camellia japonica. Restriction of water supplies to the plant during cloudy weather may lessen the problem.

Pest 5 - Psyllids on Calistemon spp.

This infestation of a species of psyllids that specifically targets callistemon was found at the Botanic Gardens during our Monday sessions. Psyllids belong to the Family Psyllidae, Order Hemiptera and have sucking mouthparts. 

Appearance : The adult insects are small (less than 4mm in length) and look like miniature cicadas.  Adults have two pairs of wings but they are not strong fliers. They are often hard to see because they jump actively. The nymphs of Lerp insects produce a shell-like covering, which gives rise to their name Lerps. The coverings are commonly white in colour, but are highly variable. Some species do not have protective covers. Psyllids are free-living and do not produce lerp coverings but secrete a white powdery wax for protection. 

Damage : The signs of damage to the callistemon is discoloured reddish and yellow patches on leaves, in some areas like blisters, due to the sucking of the nymphs and adults and the toxic saliva . In heavy attacks, infested leaves may fall prematurely. In other plants, some species can produce honeydew which gives rise to the formation of sooty mould. In the case of the callistemon pictured, the nymphs have settled on the undersurface of the leaves, with feeding causing circular lumps on the upper surface and corresponding depressions on the lower surface, with the scale-like insect inside. 

Host : In this case Calistemon spp. but lerp insects can attack other native trees, especially eucalyptus. Each species of lerps are usually quite specific in what native trees they attack. 

Life Cycle : Gradual metamorphosis with several generations within a season. Nymphs usually go through several stages before shedding its exoskeleton to form the winged adult stage. The overwintering of lerps and psyllids is unknown. Spread is usually only to adjacent plants as the adults can't fly far. 

Control : usually a tree can survive an onslaught the first time and nothing need be done. Birds and other predators feed on lerp insects. Pictured below is a predatory beetle.

With repeat attacks that severely weaken a tree it might be worth considering removing that tree and replacing it with a less susceptible species. Only councils have suitable equipment to handle appropriate insecticides which must be systemic (contact sprays are ineffective due to the lerp coverings). For valuable trees, injections of systemic dimethoate (Rogor) could be considered. 

Reference :  Kerruish, A-151

                     McMaugh, pg. 196

Biological Control of Two-spotted mite

In this activity we observed the release of a predatory mite to control a two-spotted mite infestation on Jatropha macrantha, a tropical plant, located in the Botanical Gardens succulent house. 

Mites are part of the class Arachnida, having 4 pairs of legs (although nymphs have 3 pairs). Both adults and nymphs have piercing and sucking mouthparts and feed on a variety of plants. 

These pests are located on the underside of leaves and can be just visible to the naked eye. They produce quantities of fine webbing. 

Damage to the leaves can manifest as blisters, curls, chlorosis, defoliation, abnormal pigmentation, or as in this case silvering of the leaves. Fruit can be malformed. In cold temperatures, males die but the adult females can overwinter on lower parts of main stem, in cracks or damaged bark and under debris round the base of the main stem. The hibernating adult females change colour from pale greenish or yellowish to red. Populations are greatly reduced in long periods of rain or with good irrigation. Conditions favoured are high temperatues, particularly in summer months. 

The two predatory mites available in Australia are Typhlodromus occidentalis  and Phytoseiulus persimilis, the latter being the one available through private companies. 

Reference : Kerruish, A-194

Pricking out lettuce seedlings

In this activity we used dibblers to gently lift individual lettuce seedlings from seed trays. It was important to handle seedlings by their cotyledons and not by the stem. Each individual seedling was placed in a propagation mix consisting of 50% compost, 50% pasteurised sand, 250gm of dolomite, 250gm of crushed biopellets per 50L of mixture. 

Pest 4 - Pear and Cherry Slug

Pest Name : commonly referred to as "slug" due to appearance, but is correctly the larva of a sawfly, Caliroa cerasi. (Order Hymenoptera).

Appearance of Pest : The adult sawfly is 7mm long, glossy and black. Females have a saw-like ovipositor at the end of the abdomen which is used to make a slit in leaves and deposit eggs. 

The larva or "slug" is what causes the damage. The slug-like appearance comes from the smooth, green slimy exudate what it produces whilst feeding. When fully-fed, the slime is shed and they become orange-yellow. When fully grown, the larva measures 1.3cm in length. It has a body that it slightly enlarged in front and tapered at rear. 

Life Cycle : Complete metamorphosis (the insect hatches from egg in a totally dissimilar from to adult), with 2 generations per year. Pest hibernates as larvae in soil, in spring (late Oct - Nov) adult emerges and lays its eggs in leaves. In two weeks the eggs hatch, larvae thus seen in Nov-Dec. Larvae feed for 3 weeks, then crawl or fall into soil to form pupae. Adult appear again in late Dec-Jan and lay eggs giving rise to the second generation of leaf chewing larva in summer through to autumn. They are generally much more numerous and destructive second time around. They overwinter in soil. Favours cool, moist conditions - adults can only emerge from soil if it moist and larvae can dry up quickly on leaf surface when dry. Spread is by adults flying or being carried by wind - adults can travel large distances. 

Damage : Leaves only are attacked.  In wet weather can fed on undersurface of leaves. The larvae feed mostly on upper surface, leaving veins and lower epidermis - creates the characteristic "window-paning." Leaves can turn brown, shrivel and fall. Continued heavy infestation can severely weaken trees and reduce cropping. 

Host Plants : Ornamental trees, fruit trees, shrubs and climbers : eg. pear, cherry, plum, sometimes peach, nectarine, almonds, quince, hawthorn, cotoneaster, hardenbergia

Controls : non-chemical : Control first generation - avoid overhead watering so that leaves are not wettened, slugs may be squashed manually if only a few trees, fly netting on the soil under the tree can trap emerging flies. Cultivating soil under tree can disrupt and destroy pupae. There are no biological controls.

chemical : Soap-based or Oil sprays, eg Pestoil, botanical extracts eg. rotenone (Derris Dust) - no longer recommended due to a reported increase in Parkinsons Disease, carbamates (eg. carbaryl), organophosphates. Sprays are only justified if the tree is dying or severely affected or in times of drought when defoliation is not tolerated. 

As the pest does not affect fruit, often nothing needs to be done. Natural predators include a paper wasp; birds like blue wrens, shrike thrushes, yellow robins

References :  Kerruish A117

                       natural control of garden pests, Jackie French

Propagating Ferns

Activity Tasks :  March 26th. We propagated spore collected from Dicksonia Antartica and Cyathea Cooperii. Ferns, being primitive plants, lack flowers and so reproduce by spores. Each spore case (housed in the protective sori on the leaflet of the fern frond) contains clusters of sporangia that hold the dust-like spores. To collect this fine spore, the fronds were kept previously in manilla envelopes in a warm, dry place. Spores of most temperate ferns ripen in mid to late summer. 

We prepared a growing media of two parts sphagnum moss peat and one part coarse sand of about 10 litres, mixing in about 2.5 L water - enough so that moisture could still be gently squeezed from the media. We used enough of this media to fill several plastic sealable tubs to a depth of about 3cm from the rim and then into a machine for pasteurisation. Following this we gave the tubs another gentle soak with a watering can before sealing the lids. 

We then gently sprinkled the spore over the surface of the media and again sealed the container, before placing it into a palstic bag to be tied up and stored in the propagating house. Hardy, cool-temperate ferns like Dicksonia can germinate at 15 -20 degress celsius. It can take anywhere between 2 - 26 weeks for the prothalli (the sexual, gametophyte stage) to form on the surface of the media.

OH&S issues : Sphagnum peat needs to be gently rubbed through a sieve, which can raise a lot of particle matter in the air and in large quantities aggravate the airways. As a safety measure we wore dust masks.

Environmental Work Practises/Considerations : Sphagnum peat is a finite resource and quite expensive. The peat we used was sourced from Holland. It is ideal for this type of propagation as it has an excellent water holding capacity and it has a low pH. It requires moistening before use. In the absence of a sterilising/pasteurising unit, the media can be treated by pouring boiling water over the surface. 

Ref :  RHS propagating plants

          Ferns from Spores handout

Harvesting Dutch Cream Potatoes

Area worked in : Production Garden, 25th March 

Activity Tasks : We worked with some of the organic certificate students to harvest a crop of dutch cream potatoes. Potatoes can be left in the ground where conditions permit, but the longer they are left the greater the potential for damage from rot and slugs. Generally speaking, early to mid autumn is a good to time to harvest. We used a garden fork to loosen the soil, being careful not to prick any of the tubers, and then pulling the whole plant out. Potatoes from one plant were placed into a pile to note amount, size and absence of disease in that plant. The best plants were selected to be seed tubers for the next lot of planting. 

Potatoes are best stored in the dark to prevent them turning green and developing high levels of a solanine, (a toxic alkaloid). A double layered paper sack folded loosely at the neck or a hessian bag are ideal. Ideal temperatures for storing are 5 - 10 degrees celsius. 

In the photo below there is a potato on the right with a either a bacterial or fungal rot which only infected one or two potatoes in a particular plant, while the rest of the tubers looked fine. The potato on the left is I think just really big!

White Clover (trifolium repens), a prostrate perennial herb with a rhizomatous growth habit, had been planted in rows along the beds to discourage the spread of rope twitch (agropyron repens). 

OH&S issues/PPE used : usual protective clothing to avoid exposure to sun and allergens. 

Environmental Work Practises : Leafy growth was disposed by composting and diseased potatoes were to be composted in a separate pile. It is recommended to avoid replanting potatoes in the same area for at least 4-5 years to avoid disease infestation. 

References :  RHS guide to fruits and vegetables

Using the mulcher

Activity : 

During the work practical on the TAFE campus I worked in a group of three people to feed pruned branches of allocasuarinas removed from garden beds through a mulcher. Pre-safety check of machine included checking fuel, oil and air filter and removing any stuck debris prior to starting. One person collected cuttings of similar size, bundled them and handed them to the other person who was feeding them into the funnel of the mulcher. A good tip was to never use a pair of sequateurs near the inlet as they could drop into the mulcher. The third person was on the other end, sweeping the mulch away and collecting it into a wheelbarrow. We found that we needed to feed the pile a second time to get a good sized mulch. The machine was washed down after a couple hours after it had cooled down properly. 

OH&S issues : 

Eye injury from flying wooden debris, dropping hand tools into mulcher damaging the tool and the machine blades, hearing damage from exposure to loud noise all highly likely to occur and of high potential impact. Other issues include exposure to dust and sun. 

P.P.E. used : 

To address these risks, we were required to wear protective goggles, helmets fitted with mesh masks and ear muffs, as well as gauntlets to protect hands and arms during handling and feeding of the prunings and cuttings. We also wore full sleeve shirts and long pants with steel cap boots and high visibility vests. 

Environmental Practises : 

This was a reasonable way of recycling green waste back to the garden although it does require the burning of fossil fuels. The machine we used was inefficient and created alot of noise pollution. 

Weed Control Record 1

Location :  TAFE car park side border, 12th/17th/18th March 2009.

Weather :  mostly fine and sunny, temperatures 20 - 25 degrees celsius. 

Scenario at hand : Part of work practical on TAFE campus, car park side border requiring removal of existing grasses and native shrubs, cultivation of soil and preparation for replanting. 

Previous weeding on this site :  largely unweeded and uncultivated site for several years. 

Weeds present :  Rope twitch (Agropyron repens), or English couch. Heavy infestation. An erect-growing, perennial rhizomatous plant which can spread in large patches. White Rhizomes can be a considerable length, up to metres long, spreading under barriers like concrete paths. Cultivation of soils can break existing rhizomes,which if left in soil can regrow from nodes and spread quickly. 

Treatment Options :  Basic hygiene practises such as cleaning clothing, tools and machinery that come into contact with soil carrying rope twitch is very important to avoid spread of rope twitch segments to new areas. Cultural methods suggested by the DPIW include the use of an impervious mulch layer to smother the plant and surrounding soil are, depriving the rope twitch of air, light and gaseous exchange from air to soil - eg. weed matting or black polythene sheeting. The drawback of this technique is that it needs to be laid down for several months to be even reasonably effective and can deprive the soil of beneficial organisms. A mulch layer of woodchip or bark alone is not sufficient usually to control rope twitch as it tends to grow in the mulch layer, but this makes manual weeding somewhat easier, although rhizomes will remain in the soil. Manual control includes freezing but Tasmania usually doesn't get sufficient frosts.

Chemical control can be achieved by the use of round-up or fusilade, but this will also target other monocotyledons such as iris and lillies. 

Treatment Techniques chosen : In the end we decided to manually weed what rope twitch we could using a square mouth shovel to first break up the soil and then a garden fork to bring the rhizomes to the surface for removal. 

OH&S issues :  correct technique for manual weeding to avoid back strain, clothing and broad-brimmed hat to avoid sun exposure and exposure to insects/allergens, wearing high visibility vests because of nearby traffic. 

Environmental Work Practises : cleaning of all tools after use, disposal of rope twitch by leaving it in isolation in the compost area exposed to full sun - the best technique is by solarization. 

Pest 3 - Wooly Aphid

I found this infestation of wooly aphid on an apple tree walking down the street in North Hobart.

Wooly aphid is of the Hemiptera order and its scientific name is Eriosoma lanigerum.

It is a serious pest of apple trees. They have piercing and sucking mouth parts. 

Appearance : Adults are usually always female and are mostly wingless, although some do have wings particularly in autumn, but are mostly insignificant in Australia. Globular in shape and purplish in colour, they are usually hard to see because of the white cottony threads they secrete from their pores which gives them a protective covering over the colony. They also produce a white powder which covers their bodies. Nymphs are also usually female and are smaller, initially not so globular and paler. 

Life Cycle : Gradual metamorphosis (egg - nymph - adult). Most aphids are wingless females that can reproduce asexually. The young are mostly females. Winged forms appear in autumn and can migrate to neighbouring trees. More migration occurs from tree to roots in early winter with a return to above ground parts in spring. Survival of population is usually by aphids hibernating in cracks and crevices of old wood (overwintering). 

Damage : Wooly aphids are sap suckers and target new growth where it is thin, such as lateral growth), damaged areas such as pruning areas or areas of broken bark. Over pruned and unhealed trees are particularly susceptible. Affected wood takes on a lumpy, gnarled appearance due to the sap sucking action; laterals become deformed, cracked and buds are destroyed by the woolly material secreted. Fruit can prematurely fall off or be disfigured, and the woolly secretions can make it very unpleasant to harvest fruit in heavy infestations. Roots can be gnarled and growth can be stunted in young trees. In addition, the honeydew secreted by the aphids makes conditions favourable for the growth of the fungus, sooty mould. 

Distribution on plant : aphids congregate on thin, new or damaged growth in spring and autumn but then over winter usually remain in cracks or underground in root system. Prefers cool, moist conditions and therefore will favour shaded areas of the tree, particularly in the interior of dense trees. Low humidity and high temps (> 27 degrees celsius) are not tolerated. 

Their spread is mostly by being wind-borne or by humans on clothing.

Host plants : mostly apple trees, rarely pears. Also affects hawthorn, cotoneasters and liquid amber. 

Controls :  

cultural : The use of resistant rootstock such as "Malling-Merton" and "Northern Spy."

In ornamentals, woolly aphid could be scrubbed off but this will only reduce populations.

Preventive measures include not mowing around trunk, pruning in summer when cuts heal quicker, covering wounds with an antifungal sealant paint.

biological : The wasp Aphelinus mali was introduced into Australia in the 1920s and parasitizes the wooly aphid. These aphids appear black and have the wasp larvae feeding within the aphid body, killing it. Small holes, visible to the naked eye, appear on such aphids. The wasp can overwinter as pupa within the aphid bodies and then appear as adult wasps in spring at the same time that aphid populations become active. Conserving these wasps is an important method of keeping the woolly aphid populations under control can can be achieved by not using non-specific pesticides and allowing occasional colonies of woolly aphid to exist on surrounding hosts eg. hawthorn. Twigs with parasitized aphids can be stored in sheds over winter away from birds and introduced into infested trees in spring. 

There are also natural predators of woolly aphid such as ladybirds, lacewing larvae, syrphid fly larvae. 

chemical :  organophosphate : dimethoate (but this is toxic to Aphelinus mali). Winter oil in dormant periods, maldison (another organophosphate but non-systemic), pirimicarb (carbamate).

References :  Kerruish, A148-150; Judy McMaugh, pg 213;  Natural control of garden pests,  Jackie French, pg. 79-80.

Work Placement 1 - TAFE

Area worked in :  Car Park Side Border and Main Border

Activity Tasks : 

March 11th : In a small group we removed all Poa labillardierii and  Lomandra longifolia from the bed. We used mattocks to form a circumferential trench around the grass and then levered the grasses out of the ground. We set these aside for division, propagation and planting elsewhere. We then started weeding out the rope twitch, which is extensive in this bed due to its rhizomatous growth habit and the lack of any form of cultivation in this bed for some time. We discovered localised areas within this bed of very poor drainage and water logging. The soil dug up in these areas had a nasty smell indicating quite poor anaerobic conditions. In addition, all the native pelargonium was pulled out using a pitchfork. We had not previously located and isolated the irrigation system in this bed and I caused a puncture in the poly pipe which required repair the following day. 

March 12th : With assistance from Roy Baker, we repaired the puncture in the irrigation system. This was done by first digging a trench with a shovel and exposing the site. We cut the section of pipe with a hand saw and later Roy obtained replacement piping which we cut to size, filing the edges and attaching to the two ends with plastic couplings. This was a cheap and reasonably quick process. We then flushed the entire section of the irrigation system to remove internal debris and cleaned the filters on each sprinkler. 

March 17th : We continued the process of manually weeding out the rope twitch present with a pitch fork and exposing the irrigation system fully to prevent further damage to it. We then carried out some pH testing using battery operated metres. These metres were calibrated first with known neutral (pH 7.0) solution. A 20g sample of soil was then mixed thoroughly for

 about a minute with 100mL of distilled water, making sure no organic matter contaminated the soil sample.  The meter was placed in the solution, being careful to avoid placing the meter in the soil sediment. To measure the pH we tested a couple of different sites in the bed instead of doing a composite analysis (soil samples taken from many different points in the one site and mixed together). This was because the soil profile at various sites demonstrated it was not 

an homogenous area.  Results are seen in the photo below. 

In the main border there were mixed pH readings of 6.3 and 7.4 where the westringias and hakea had been growing respectively. In the carpark border, pH 6.6 in the clay loam and 6.3 in the sandy loam. In the car park side border, pH 7.3 where a sad acid-loving camelia was growing and 6.4 on the other end. 

Soil texture analysis in this bed showed areas of sandy loam to clay loam.  

March 18th and 19th :  Worked on the main border where turf had been removed for a new section of this border. This area had a hard clay based soil which required breaking up first with a square mouth shovel. Shallow root masses were removed with a garden fork and then a pick-ended mattock was used to break up the clay even further. This was quite a physical exercise and proved more efficient when we worked in sequence, one person following on from the other person in a given area. We then leveled the bed as best we could with a metal rake, using the bottom end to level and the toothed end to break up clods and rake up surface debris. The last job was to place compost which had been previously calculated by the cubic metre in piles one metre apart and then spread by metal rake over the bed to a thickness of about 100mm. 

OH&S issues :  Constant issues included presence of cars and general public accessing area. These were addressed by wearing high visibility vests, placing signs and fluorescent witches hats around the area. There were no underground electrical issues in this bed. Holes left behind following the removal of the grasses became an issue and had to be addressed by leveling these holes. Risks of exposure to sun, potential allergens and insects was managed by wearing the appropriate P.P.E. Unused tools were placed in a corner leaning on a wall cl

ose to but away from the work site to prevent injury. Risk assessment sheets were completed for each day. Examples in photos below : 

P.P.E. : full sleeve shirt, broad brimmed hat, glasses, steel cap boots, long trousers, fluorescent vests. 

Environmental Work Practises : Proper disposal of the rope twitch had to be thought about to limit its spread. We used separate bins for the rope twitch and transported this to a separate section of the compost area. Solarisation by exposure to full sun is a safe and environmentally sound method of disposal of rope twitch. We ensured that we cleaned all equipment that we used prior to returning them to prevent spread of the weed. 

Reflection : It's important to know the location and isolate any underground piping or cables in an area being cultivated. We could have asked for the plans for the irrigation system before we started digging up the area. We could consider using the herbicide fusilade (targets monocotyledons non-specifically) but this would make it difficult to grow other grasses in the bed. Working efficiently means knowing how to use a hand tool effectively, with the least amount of physical exertion and working in a co-ordinated fashion. 

Field Texture Test

In this short practical we practised two methods which can determine a soils texture by hand. It is simple and very useful. 

Method : 

1. take a small sample of the soil to fit in the palm of the hand, discarding any gravel bits. 

2. moisten soil with water, little by little, and start to work the soil in the hand like dough until there is no apparent change in feel. This usually takes several minutes. 

3. Inspect the sample to see if any sand is present. Rub a little of the sample next to ears to listen for the sound of sand grains. 

4. Squeeze sample hard to see if it forms a ball (cast). See whether the cast holds or falls apart. 

5. Finally squeeze sample between thumb and first finger to form a ribbon using a sliding motion. Note the length of the self-supporting ribbon. 

6. Refer to table for a suggestion of its texture (ranges from sand to heavy clay). 

On this testing, the patch of soil used was from the top vegetable growing area and seemed to fit the description of a sandy clay loam. 

pH testing with colorimetric test kit

Today we analysed mudstone, dolerite, basalt and limestone based soils collected from soils excursion exercise yesterday.

pH testing was performed with the use of a colorimetric kit, a quick, portable and reasonably cheap method of approximating soil chemistry. A sample of soil no bigger than the size of a 20 cent coin was placed on a white tile, removing large bits of debris and organic matter. Indicator solution is added and mixed until a thick flat paste is formed. White barium sulphate powder is then lightly sprinkled on the surface and after a minute the change of colour to the edges of the powder applied is compared to a colour chart. This chart is accurate to the nearest 0.5 and is an approximation only. More accurate readings can be obtained using an electronic meter. Probes are useless. 

Only major OH&S issue is to prevent inhalation of the barium sulphate powder. Environmental issues that would need to be considered include disposal of chemically treated soils - would need to do further research on potential impacts of different methods. 

Results :  dolerite pH 6.0, mudstone pH 5.0, limestone pH 6.5, basalt pH 6.0

These results were reasonable, but probably not accurate due to the presence of other matter in the samples. Dolerite and basalt are igneous rock which is high in nutrients and generally more alkaline with typical range of pH 6.2 - 6.5.  Mudstone is a sedimentary rock which is low in nutrients and more acidic with a typical range of 5.0 - 6.0

The use of the colorimetric kit requires practise, especially to reliably match colour changes. A thin layer of barium sulphate scattered evenly and left alone is essential. Interpreting results and comparing colour changes seems to require some experience. 

Following this practical in the classroom I performed further pH testing on soil at home (photos above show the steps). I analysed various soils sites and consistently had readings of between 6.0 - 6.5

Disease 2 - Powdery Mildew on Cucurbits and Rose

Above are two photos of powdery mildew on pumpkin.

Below are photos of infection of a climbing rose : 






Powdery Mildew applies to a group of related fungi. 

Host Range : 

Generally a particular species of powdery mildew is restricted to one host, or a group of related hosts. Ornamentals, fruit, field crops and weeds are affected. Roses, apple, cucurbits, grape, strawberry, peas and beans are commonly affected. 

Symptoms : 

Almost all begin as faint white spots on leaves. The spots gradually increase in size until the whole leaf is covered in white powder. Buds, stems and fruit are also affected. 

Leaves that are fully formed are not changed, but young leaves can curl up or pucker. 

Disease Cycle :

Powdery mildew spores generally require high humidity for germination. Some flourish under dry hot conditions, but germinate when there is night dew. They do not germinate in rainy weather. Once it gets going, can continue to grow even in dry conditions. Optimal temp range for infection is 11-28 degrees celsius. 

Control : 

cultural - providing good ventilation through adequate spacing, choose the least susceptible varieties, avoid overhead watering, don't water in evening in favourable conditions. Prune off all infected shoots and burn them. 

chemical  -  in commercial crops, spray as soon as disease is noticed with wettable sulphur (2-4g per litre of water). Do not use in hot weather (scorch) or in dehydrated plants. Also systemic fungicides.