Gerbera Diesases & Cure

7/31/2019 Gerbera Diesases & Cure

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AphidsAphids, commonly called “plant lice”, are insects, whose adults have varyingdimensions of about 1,5-4,0 mm in length, possess an almost transparent thinexternal integument, in colours that range from clear yellow to black.

Aphids with and without wings

They have a strong polymorphism between the winged forms, whose task is toreach new plant hosts up to a distance of hundreds of kilometers away, and thoseapteral (wingless) that, with the loss of wings and wing muscles, assemble all of their resources in the reproductive function, producing in a short time numerouscolonies.

Aphids possess piercing-sucking mouth-parts (rostrum) through which they piercethe most tender tissues (vegetable apexes and young leaves) from which plant sapis withdrawn. This is rich in sugar but poor in amino-acids, for which reason theyseek to retain the necessary nutritional elements and to expell the surplus waterand sugars in the form of sticky honeydew, through a posterior appendix of theabdomen (siphuncle).

Aphid with egg

In nature, these Homopterons have a very complex biological cycle; in greenhouses,on the other hand, they have a simpler cycle because the insect has at it’s continualdisposal a vegetable host (Gerbera) and a mild climate. All of this allows it to haveonly parthenogenetic generations, with a phenomenal ability to multiply. Onaverage a female during her life cycle produces from 50 to 100 progeny, most of



which other females that, within 8-10 days of their birth, are sufficiently mature tobegin reproduction.

The damage caused by these can be direct and indirect.- direct damage consist in the suction of plant sap, lesions and necrosis of the

tissues subject to puncture by the insect. They also produce gall and hyperplasia,because of the effect of different substances which are injected into the plant,including Indoleacetic acid (vegetable growth hormone).- indirect damages however, consists of the excretion of honeydew thataccumulates on the foliage of the plant. This honeydew supports the growth of black sooty-mould fungi, wich often renders the effected plant unsightly and theirflowers unsalable (the photosynthetic activity of the plant is hindered).

Aphid eggs

Further indirect damage is causes by their capacity to transmit viruses(carriers)from healthy to sick plants (the viruses can be transmitted even from infectedplants that live outside the greenhouse).From this point of view one of the most dangerous carriers is the aphid MizusPersicae.

Control of Aphids

Aphids usually do not cause serious damages to the Gerbera, firstly because theyare kept under control by other insecticidal treatments. Furthermore, their activityas carriers is also negligable because the plantlets that originate in the laboratoryare unlikely to be infected by a virus (micropropagation).

In cases in which it is necessary to resort to chemicals (hot environments), we canuse insecticides such as:

Pirimicarb – 200 gr/100 l ( 17.5% c.a.).It acts via contact and asphyxia, withtranslaminar properties (*)

Imidacloprid (Confidor) – 50-75 ml/100 l ( 17.8% c.a.).See control againstAleyrodidae (*)



Methomyl – 150-200 ml/100 l ( 19% c.a.). Cytotropic insecticide, it acts via contactand ingestion. It is largely used also against Thrips, Aleurodids and Lepidopters. Thelatter also possess an ovicidal activity, and it is used for the preparation of poisonedbait (*)

(*) Product of prevalent adulticide activityOne or two treatments are generally enough to control the insect.

Grey mildew (Botrytis Cinerea)

Pathogen: Botryotinia Fuckeliana

Conidial Form: Botrytis Cinerea

Class: Ascomycetes

Order: Helotiales

This is a pathogen which affects a large number of plant species, among which theGerbera unfortunately is one of the most susceptible, especially in periods or verydamp cultivation conditions. From this point of view it is undoubtedly (together withroot rot) one of the diseases that limits te cultivation of the Gerbera in many areas.

Grey mildew on a plantlet

The disease, commonly known as Grey Mildew, typically causes a “soft rottenness”on all the leaves that are covered (in severe cases) with large amounts of greymildew. This grey mildew consists of the multiplication or conidial forms of thefungus. The mycelium and sclerotium are their resting structures; these live in thefoliage remains on the soil, or on wild plants (Amarantus Genus). Botrytis spores(conidia) contain very little water so they depend on the dampness of theenvironment.

They germinate extremely well even in a thin layer of free water, especially if itcontains sugars. Thus, condensation on leaves is perfect. A short dry period (about2 hours) does not harm the germinating spores, but longer periods (several hours)



of dry conditions will dry out and kill them. When the leaves are wet and thetemperature is 15-25°C, the spores germinate and grow.

They form mycelium threads that penetrate the leaves, which marks the start of infection. Once the disease is established it forms new spores, especially under

humid conditions. Spores are released when the spore-bearing plant material isdisturbed or when the relative humidity (R.H.) changes rapidly (in the morning andlate afternoon).

Under normal conditions (low R.H.), unless an abundant innoculum source isavailable, the fungus is incapable of attacking vital organs and in activedevelopment. These, in fact, are damaged only if they are come into contact withan organ already infected with abundant grey mildew.

The pathogen develops in abundance on old organs, producing a big quantity of enzymes that damage the adjacent vegetable tissue and facilitate the invasion of the fungin mycelium. On the young Gerbera plants, the rottenness produced at thebase can cause their death with notable economic repercussions.

When fungus attacks young flowers it causes their death, and thus reducing production

On the flowers, the most characteristic symptom is the appearance of brown sunkenspots of various dimensions; when climatic conditions are not too negative, they arenot very visible to the naked eye during the harvest. However, these spots growbigger after the harvest, during the storage and packaging becoming so noticableas to make them unmarketable.

The fungus is so virulent that it can also cause damage in high summer if atnightime the relative humidity in the greenhouse reaches values of about 95-100%. The infection is encouraged by temperatures between 4 and 32°C, but occursespecially when R.H. levels in the greenhouse are higher than 85-90%. The spreadof the pathogen takes place through water and air.



Typical Botrytis “spots” on a flower

Control of Botrytis To fight this dangerous pathogen we should adopt all those agronomic practiceswhich reduce the occurence of the conditions of predisposition for the disease, suchas: the reduction of films of water on the plants; reduced high humidity, and thereduction of especially luxuriant and etiolate plants.

To prevent Botrytis damage we must make use of:Heating with pipes of warm water placed at the base of the plants

Irrigated systems that do not wet the foliage, especially during the autumn-winterperiod

Openings at the top of the greenhouse

Heating systems with pipes of warm water positioned between the plants and theopenings at the top



Leaf minerThe leaf miner, a small fly belonging to the Agromyzedae family, was first introduced into Europefrom America, probably through chrysanthemum plantlets towards the mid 70s.

In Italy it’s presence was first noted in 1978. Currently in many countries it represents, together withThrips parasites that effect the Gerbera.

It is a polyphagous insect, capable of infesting numerous horto-floro plants and weeds. Amongst leaf miners the Liriomyza T. is undoubtedly most troublesome to growers, both because of itsvoraciousness, and it’s ability to become resistant to insecticides.



The adult leaf miner is a fly of small dimensions (2,0-2,5 mm), it has a yellow head, a blackishthorax, and a clear yellow abdomen with clear brown striped wings. The larva are a yellow orange

colour, whilst the pupae are small firm brown-yellow pods about 2 mm, usually visible on the upper leaf surface. The male is smaller than the female, they both usually fly about during the morning, andthey mate in the daytime.

The virgin females cannot lay eggs if not fertilized by the male. These, by means of a small thread, pierce the leaf epidermis on the upper side, causing lacerations from which vegetable sap comes outwhich also acts as nourishment for the adult male unable to pierce the leaves. From the yellow-creamcoloured eggs the yellow larva emerges. This feeds by digging a tunnel-serpentine mine- in the leaf mesophyll, narrow in the first few centimetres but becoming wider and wider, ending with a largecircle. If you hold the leaf against the light you can see along the whole length of the tunnel a

blackish line made by the larva excretions. If the larva is not present in the tunnel, the larva’s exithole can easily be seen in the shape of a half moon at the point where the tunnel opens into a largecircle.

From the larva comes the pupa that very often, in cases of strong infestation, is visible because itrests upright from the point where the leaf has been perforated. The cycle is completed with thetransformation of the pupa into an adult.



The duration of the cycle varies according to the temperature, with optimal values, of 27 – 28 C thecycle can be completed from egg to adult in about 14-16 days.

At temperatures higher than 30 C, larval mortality gradually increases. At optimal temperatures, thelarval phase is much shorter, lasting from 4 days at 30 C to 7-8 days at about 20 C.

The adult leaf miners live on average 13-30 days, depending on the temperature, the females livelonger. The male/female ratio is generally balanced. According to laboratory research the followinghas been noted: at 13 C the insect does not reproduce, at a constant 14 C the eggs openings block,and only at about 16 C do the adults start merging from the pupa.

Observation made in different greenhouses also show, that in cold environments the insect livesthrough at least one generation during winter, while in well-heated greenhouses numerousgenerations follow each other in succession.

Larva Liriomyza Trifolii Pupa Liriomyza Trifolii

The principal damage is caused by the larvae that dig tunnels in the leaf mesophyll, also considerablyreducing the plants’ photosynthesis and thus the growth and production. In cases of stong infestationthe leaf miner is also able to lay eggs inside the ligulate flowers.



We must remember, however, that the leaf miners Phytomiza Horticola and Phytomiza Syngenesiae,known to the floriculturists as “”Old miners”", today have been completely supplanted by LiriomyzaTrifolii.

The old miners, grey-black coloured, have pale yellow larvae. The pupa are found like the

LiriomyzaTrifolii at the end of the broadening tunnel dug in the leaves. However, unlike theLiriomyzaTrifolii, the4 pupae are located inside the leafrather than above it’s surface. This, and thewinding tunnel which they produce, distinguishes the old miners from the Liriomyza Trifolii.

Eggs Leaf miner with larva inside



White powdery mildew

White powdery mildew, a fungus, manifests itself initially on the leaves, petioles and the flower heads, with the appearance of a characteristic whitish efflorescent, floury, constituted by theabundant formation of mycelium and conidial multiplications of the fungus.

Following such infection the leaves become smaller, curled up and of leathery consistency. In a short period the white efflorescence turns into a purplish brown colour. Following this the affected organsdry up and die.

The flowers often do open and necrotic spots appear on the stems. Even if the pathogen does notcause the death of the plant and chemical control is not carried out in time, the total deterioration of the flowers will be the result. The mildew flourishes in warm-damp conditions, but the ability togerminate and to spread, even when the relative humidity is low, and a broad range of temperature – min 3-4 C, opt 20-25 C and max 35-36 C – makes this pathogen dangerous in any period of the year.The fungi resting structures during winter are the cleistothecia that in spring first release theascospores and from these the conidia. These are transported by wind and reach the leaf surface of the Gerbera on which they will germinate and produce the disease. Not all Gerbera varieties areaffected in the same way. This depends on the susceptibility of individual varieties. White powdery

mildew generally appears in small amounts and then usually spreads quickly and widely on the samevulnerable variety. It is always essential to act quickly at the first signs of the infection.



White fly

The white fly (T. Vaporariorum) and more recently the Bemisia Tabaci have become amongst the

most dangerous phytophagous in greenhouses both for the great polyphagia and for their highreproductive capacity, finding in the warm-moderate areas and in greenhouse environments, optimalconditions for their development.

The insect improperly defined together with the B. Tabaci is considered to be of Central Americanorigin. The adult measures about 1 mm. It has a wax-like body similar to white powder, which givesthe insect its characteristic white-yellow appearance. The female is bigger than the male, and it laysthe eggs in short stalks, on the undersides of leaves in groups from 20 to 40.

Initially the eggs have a clear yellow colour, but after a couple of days purplish.

The whole biological cycle occurs on the underside of the leaf, and is divided into 7 stages; the cycleincludes 4 nymphs stages, an intermediate stage called pupa, during which ocular red spots arevisible, and finaly the adult.The stage 1 nymph is of a clear-green colour and initially mobile, subsequently it is able to insert it’s

piercing sucking mouth parts into the leaf and it becomes sessile.

They are flat, appressed closely to the leaf, oval in shape, resembling pale green scales and havewhite, waxy threads, radiating from their bodies.

All other stages, excluding the adult, live attached to the leaf.

By making an opening on the back of the pupa the adult white fly is able to emerge, and immediately begins to feed. The virgin females lay only haploid eggs from which give birth to males only. Withthe mating process. On the other hand, both males and females are produced, with the femalesdominating in numbers.



Mating occurs 10-12 hours after leaving the pupa, and it continues for the whole duration of the adultlife, for an average of 60 days. During their reproductive cycle the females can lay from 150 to 500eggs. The number of generations is directly related to the temperature, below 10 C the white fly doesnot reproduce, it has its optimum temperature of around 29 C to 30 C, and above 34 C the cycle isunlikely to be completed.

These Aleyrodidae, live together in high numbers in a very sociable way on the under-sides of leaves. They are damaged by the sucking of plant sap, the production of abundant honeydew with theformation of sooty-mould, and as virus carriers.

In cases of serous infestation, the plant loses vigor and a general yellowing of leaves is noticed, dueto the dry cell content.



Thrips

Thrips are insects of tiny dimensions, whose females have a morphological thread by which theydepose the eggs inside vegetable tissues. The Frankliniella Occidentalis or California Thr., found inEurope in Italy for the first time in 1987 on carnations, has become in a few years, for all EuropeanGreenhouses and in the moderate climates of other countries worldwide, one of the most dangerous

phytophagouses of the Gerbera and many other floral cultivation.

Adult Frankliniella Occidentalis

Present in many parts of the world, similar to a midge of a pale yellow colour, it is also unfortunatelyfamous for the ability to transmit the dangerous virus of the Tomato Spotted Wilt Virus (TSWV),able to infest more than 300 species of vegetables.

The male is on average 0,9-1,0 mm long, while the female is slightly bigger (1,2mm). The eggs areinvisible to the naked eye and the life cycle is divided into six phases of development: Egg – 1.larvalstage – 2. larval stage – Prenymph – Nymph-Adult.

The egg is laid in the parenchymal cells of the flowers and of the leaves, and after a few days theseopen and a larvae immediately beginning to feed on the parenchymal cells and on the flower pollen.During the nymph 2 phase, they grow and their mobility increases.

The Prenymph and the Nymph complete the cycle. They spend the whole winter in shallow soil or residual vegetation.

The adults live on average 30-40 days and the females-male ratio is generally 3:1 or 4:1.

The Thrips, few days after the eggs open become very active managing even to make jumps of3 cm.

Because of their small dimensions, they can also be spread by the wind. The females begins to layeggs within the first 3 days of her life, the unfertilized eggs produce males, the fertilized eggs

produce mainly females. Generally 1-2 eggs are laid per day, reaching a total of about 40-50 eggsduring the whole life cycle.



In good conditions, like heated greenhouses or the right natural climate an insect can have up to 12-14 generations in a single year.

Nymphs of I + II stage LNymph of II stage

The damage to the plants is caused by the I and II age Nymphs, and by the adult Thrips.

These have rasping mouth parts that abrade the surface of flower petals and leaves that release plant

sap, which is then sucked up. This rasping injures the plant tissue, leaving brownish streaks an lightcoloured flower petals, or whitish or silvery streaks on foliage or dark coloured flower petals. To thisdamage the action of the terebra can also be added, which causes great damage especially to thesmall buds. In case of large outbreaks, the younger damaged leaves are deformed and of silverycolour. The silvery colour appears due of the emptying of the epidermal cells by the sucking actionof the parasite. As well as such directly caused plant damage, the insect is also able to transmit andspread dangerous viruses from a single infected Gerbers plant, and also from wild plants situatednear the Gerbera plants.

Thrips Tabaci, on the other hand is native to the Mediterranean areas, and often found on the Gerberatogether with the Frankliniella O. Compared to the latter, the Thrips T. Lives more on the plantsurface, thus is more vulnerable.

An other common species, which affects the Gerbera is the Heliothrips Haemorrhoidalis. If present ingreat quantities, it can be recognized because it lays its eggs on the underside of the leaf, close to themain vein. In addition, this egg laying is unusual to this insect, since females cover the eggs with

blackish drops of excrement.

Thrips attack on flowers Different degrees of Thrips attack



Leaf spots

Leaf spots, a fungus, can, in some countries, also cause severe damage to the leaves whereto beginwith circular shaped spots with irregular borders appear, subsequently they become bigger and insidethey have a series on concentric circles diminishing in size, and with prominent borders.

Generally of a purple-brown colour, leafspots in more serious cases have the tendency to gather together, forming large necrotic areas that subsequently cause the leaves to whither. These pathogensspread by means of the conidia and spores, and flourish in hot and highly humid conditions.Outbreaks of Cercospora can be distinguished from those of Alternaria by the holes found near thecentre of the necrotic areas.

The controls of these diseases are primarily of an agronomic nature. These controls seek to avoidhigh levels of humidity, and to encourage good physiological and sanitary conditions for the plants.

Leaf spots on gerbera

Necrotic leaf area probably caused by Alternaria

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Gerbera Diesases & Cure