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 the Gerbera unfortunately is one of the most susceptible, especially in periods or very damp cultivation conditions. From this point of view it is undoubtedly (together with root rot) one of the diseases that limits te cultivation of the Gerbera in many areas.

Grey mildew on a plantlet

Grey mildew under a microscope

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 plantmaterial is disturbed or when the relative humidity (R.H.) changes rapidly (in the morning and late afternoon).

Under normal conditions (low R.H.), unless an abundant innoculum source is available, the fungus is incapable of attacking vital organs and in active development. These, in fact, are damaged only if they are come into contact with an 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 the base 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 sunken spots of various dimensions; when climatic conditions are not too negative, they are not very visible to the naked eye during the harvest. However, these spots grow bigger after the harvest, during the storage and packaging becoming so noticable as to make them unmarketable.

The fungus is so virulent that it can also cause damage in high summer if at nightime the relative humidity in the greenhouse reaches values of about 95-100%. The infection is encouraged by temperatures between 4 and 32°C, but occurs especially when R.H. levels in the greenhouse are higher than 85-90%. The spread of 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 practices which reduce the occurence of the conditions of predisposition for the disease, such as: the reduction of films of water on the plants; reduced high humidity, and the reduction 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-winter period
• Openings at the top of the greenhouse
• Heating systems with pipes of warm water positioned between the plants and the openings at the top

Source: p. 131 & 132 of “Gerbera Cultivation in Greenhouse” by Gerardo Mercurio

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Aphids

Species:

Aphids spp.

Order:

Rhynchota

Suborder:

Homoptera

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

Aphids with and without wings (Aphis Gossypii)

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

Aphids possess piercing-sucking mouth-parts (rostrum) through which they pierce the most tender tissues (vegetable apexes and young leaves) from which plant sap is withdrawn. This is rich in sugar but poor in amino-acids, for which reason they seek to retain the necessary nutritional elements and to expell the surplus water and sugars in the form of sticky honeydew, through a posterior appendix of the abdomen (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 continual disposal a vegetable host (Gerbera) and a mild climate. All of this allows it to have only parthenogenetic generations, with a phenomenal ability to multiply. On average 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 to begin 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 that accumulates on the foliage of the plant. This honeydew supports the growth of black sooty-mould fungi, wich often renders the effected plant unsightly and their flowers 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 infected plants that live outside the greenhouse).
From this point of view one of the most dangerous carriers is the aphid Mizus Persicae.

Control of Aphids

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

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

• Pirimicarb – 200 gr/100 l ( 17.5% c.a.).It acts via contact and asphyxia, with translaminar properties (*)
• Imidacloprid (Confidor) – 50-75 ml/100 l ( 17.8% c.a.).See control against Aleyrodidae (*)
• Methomyl – 150-200 ml/100 l ( 19% c.a.). Cytotropic insecticide, it acts via contact and ingestion. It is largely used also against Thrips, Aleurodids and Lepidopters. The latter also possess an ovicidal activity, and it is used for the preparation of poisoned bait (*)

(*) Product of prevalent adulticide activity

One or two treatments are generally enough to control the insect.

Source: p. 138 & 139 of “Gerbera Cultivation in Greenhouse” by Gerardo Mercurio

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