Powdery mildew disease and recent advances

Powdery MildewErysiphe necator; (Uncinula necator)

introductionGrape vine is one of the orchard, horticultural crops which adds much of the export income to the country as commercial fruit crop and also as one of the healthy dietary food to the people.

Such a crop, which is a boon to the growers is facing threat with few important diseases like powdery mildew, downy mildew, etc…

Am going to present on the topic grape vine powdery mildew symptomology and management

SYMPTOMOLOGY – IN GENERAL

• The fungus, Uncinula necator, is found mostly on the leaves and fruit stems, but it will also infect green fruit in some seasons.

• Affected berries have a dull, darkened appearance and are usually covered with the light powdery growth of the fungus.

• In some varieties new fruit infection will cease as the sugar content rises and the fruit matures.

• Infection on leaves appears conspicuously on the upper sides as white, powdery patches.

• Later, the entire surface of the leaves may be covered with the fungus.

• They turn an ash grey colour and quickly become covered in spores

• When infection is severe, the leaves will scorch or turn brown and fall.

• When grape cluster stems become covered with the fungus the stems shrivel and considerable losses from shelling of the fruit may occur.

• Black specks of the overwintering spore-forming bodies can be seen in the older affected areas.

• Infection usually starts soon after the blossom period and will continue on the foliage through the growing season.

• The fungus develops in relatively dry conditions when the relative humidity is high.

Weather conducive to disease development

Symptomology in specific

Leaves: The first powdery mildew lesions are frequently found on the undersides of leaves

(a). As the epidemic progresses, lesions become apparent on the upper sides of leaves as well.

These lesions will increase in size and number if the disease is left unchecked.

Severely infected leaves may become brittle and drop off. Starting as early as late July, very small orange to black spherical structures called cleistothecia develop on the upper and lower surfaces of leaves

Powdery Mildew Symptoms

The first powdery mildew lesions are frequently found on the undersides of leaves

• (b). Shoots: Brown to black irregular blotches that can measure up to a few centimetres, follow the gradual degeneration of the fungus over the course of the season

• (c). The spots have indistinct margins and remain visible even following shoot lignification.

Very small orange to black spherical structures called cleistothecia develop on the upper and lower surfaces of leaves

• Inflorescences and rachis: Usually seen on rachis, powdery mildew has the appearance of a grey to whitish powder.

• Severe infections of the rachis can result in clusters being dropped, especially if mechanical harvesting is done.

• Symptoms on the rachis are similar to those on shoots

• The gradual degeneration of the fungus over the course of the season

Berries: Berries can be infected from immediately after bloom through 4 weeks post-bloom.

They turn an ash grey colour and quickly become covered in spores

(d), giving them a floury appearance. At the end of the season, cleistothecia also appear on the berries

(e). Affected berries dry out and may drop off

(f). Berries that infected later during the period of susceptibility are prone to splitting, making them susceptible to infection by Botrytis.

Cracking of the fruits

• Affected berries dry out and may drop off

Epidemiology

• E. necator is an obligate parasite of grapevines, i.e., it can develop only on living grapevine tissue.

• In our climate, E. necator overwinters as cleistothecia, structures containing ascospores (or sexual spores).

• In the spring, the ascospores mature and infect the leaves growing in proximity to bark.

• Following infection, spots covered with asexual spores, called conidia, develop on the leaves.

• The conidia of E. necator do not need free water on the tissue to infect it.

• However, high relative humidity promotes germination of the conidia and therefore infections.

• Powdery mildew of grape is promoted by hot (optimum temperature of 25°C), dry (but humid) weather since water inhibits germination of the conidia.

• Cleistothecia also appear on the berries

Effect of temperature on the development of grape powdery mildew

Temperature of leaf*(oC)

Days for spores to develop and infect vine parts and produce

new spores

6 32

9 25

12 18

15 11

17 7

23 6

26 5

30 6

33 (for at least 3 days) 0 (but 10% can recover in 5 days)

40.5 (for at least 6 hrs) 0 (kills the fungus)

Severe powdery mildew infection on 'Chancellor' grape leaf, with developing cleistothecia.

Spore (condia) production of powdery mildew on a grape leaf.

Powdery mildew cleistothecia on grape shoot.

Circinate structures of fungus magnified under microscope (cleistothecium and ascus containing ascospores).

management

• clean cultivation of vines or destruction of all diseased parts.

• Grow resistant varieties etc.

• Prophalactic spray with bordeaux mixture 1% or lime sulphur before onset of monsoon i.e., before the onset of favourable conditions.

• Spray wettable sulphur @ 0.3% or dinocap or tridemorph @0.1%

Cultural Control:

• Manage canopies to increase air drainage and light penetration by removing lateral shoots in dense canopies. If necessary remove leaves in the fruiting zone. Dense canopies provide low light intensity, which favours powdery mildew development;

• Use an under-vine irrigation system (drip or micro-jet);

• Manage irrigation carefully. Excessive irrigation leads to excessive vigour and higher disease potential.

• Select varieties that are less susceptible to mildew

Prophylactic measures

Dusting of vines with 300 mesh sulphur.• 1st dusting when shoots are 2 weeks old.• 2nd dusting prior to blossoming.• 3rd dusting when fruits are half ripe.

• Spray :oxythioquinox (morestan) @ 0.03% at 4 days interval starting from last week of december to 1st week of march.

Timing Fungicide Sprays:

• Protect grape foliage from primary infection by application of fungicides from early shoot growth until after bloom. Good control early in the season to prevent establishment of the disease is the key to preventing a powdery mildew epidemic later in the summer.

• Apply fungicides such as Kumulus (sulphur), Nova, Lance, Pristine, Sovran, Flint, Milstop or Serenade at the following growth stages (also see “Fungicide Notes” below for more information).

• When new growth is 5 to 10 cm long.

• Just before or immediately after bloom.

• Every 10 to 14 days until grapes begin to soften and red varieties begin development of color and white varieties change from green to white or yellow. If Kumulus (sulphur) is used, shorten the spray interval to 7-10 days.

Fungicide Chemical Group1  Rate/ha Rate/acre PHI2 (days) Notes

Vivando (metrafenone

)U8 750 g 300 g 14

Apply at 14-21 day intervals; use shorter

interval for high disease pressure or rapid growth phases. Do not

apply more than 2 sequential

sprays.

Quintec(quinoxyfen) 13 300 ml/ha 122 ml/acre 14

Excellent mildew fungicide. Apply

on a 14 day interval. Do not

exceed 5 applications/

season. Alternate with

other fungicides. See Fungicide

notes.

Flint(trifloxystrobin 50% WG)

11 105-140 g/ha

43-57 g/acre 14

Excellent mildew fungicide. Apply

preventively using a 14-21 day interval. Do not use Flint or other group 11 fungicides more than 2

times per season. Alternate with fungicides from other groups. See

Fungicide notes.

Sovran(kresoxim-

methyl 50% WG)

11 240-300 g/ha

100-122 g/acre 14

Excellent mildew fungicide. Apply at 14-21-day intervals. Do not use Sovran or other group 11 fungicides more than 2

times per season. Alternate with fungicides from other groups. See

Fungicide notes.

• Otherwise: if a farmer cannot recognise at early stages…

It is recommended to go for a spray with Wettable sulpur @ 0.2 % Or karathane @ 0.07%

Scouting• Grape varieties: Monitor susceptible varieties.

• When: Start looking for powdery at about 3-5 leaves and continue throughout the season.

• Where: Throughout the vineyard.

• How: Monitor for the appearance of discolored spots that turn whitish on the upper and lower leaf surfaces.

Do not confuse with pesticide residues.

When checking, change the angle of the leaf.

The whitish spots are particularly visible when the leaf is held at an approximately 30° angle.

Be sure to sample leaves from the interior of the canopy since disease development is favored by shade.

HEALTHY GRAPE BUNCH

THANK YOU

http://www.ipm.ucdavis.edu/DISEASE/DATABASE/grapepowderymildew.html.