powdery mildew of apple

sexual reproduction

- asci and ascospores produced in chasmothecium - fungus can overwinter as chasmothecium

powdery mildews

- biotrophic, chasmothecial ascomycetes - most common, conspicuous, widespread, and easily recognizable of plant diseases

disease development on leaves and fruit

- colonies appear as white, felty-like patches on the lower leaf surface - leaves curl at edge, crinkle, and fold - infection on young fruit cases net-like russeting and discoloration - leaves are lesssusceptible as they mature

infection process

- conidia germinate, pulling moisture from the air, and flatten to form appressoria - enzymes help the penetration peg to penetrate plant epidermal cells - pegs enlarge to form haustroia - nutrients are translocated back out to support mycelial growth

asexual reproduction

- conidia produced in chains on short conidiophores on the plant surface - conidia are disseminated by air currents - spores germinate and cause infections under high humidity (leaf wetness period not needed)

environment

- extremely low temps impact survival as infected buds are more vulnerable to winter kill - conidia germinate at high relative humidity (>70%) - wetness is a deterrent to infection

chemicals

- foliar fungicides (7-10 day) intervals from the tight-cluster stage until terminal shoot growth ends (mid-summer) - rotate classes with different modes of action or tank-mix to prevent fungicide resistance - thrives in dry seasons, sprays can't be relaxed

powdery mildew of apple

- found in all apple-prpducing regions - chasmothecial ascomycete - biotroph

disease development

- fungus survives as mycelium in dormant flower and shoot buds - in spring, fungus resumes growth and colonizes young, green shoots (primary infection, silver-gray, stunt, dieback, defoliation)

conidia

- hyaline - formed continually - wind-dispersed - do not need free water to germinate

cultural management practices

- remove primary inoculum - chemicals

secondary infection on blossoms

- silver-gray, open later than healthy ones (if at all) - petals are distorted and pale - blossoms shrivel and fail to produce fruit

management

- successful control based on grower education and a strategy that combines resistant apple cultivars, cultural practices, and effective fungicide use

the genera are distinguished by:

- the number (one vs. several) of asci per chasmothecium - morphology of hyphal appendages growing out of the wall of the chasmothecium - characteristics of the anamorph (conidial stage)

pathogen

Podosphaera leucotricha

what kind of plants does it infect?

all kinds - most notably cereals, cucurbits, ornamentals, and trees

colonies of mycelium on the surface

appear powdery

hosts

apple (Malus spp.) and pear (Pyrus spp.)

sulfur fungicides

broad-spectrum or multi-site compounds

remove primary inoculum

flower and shoot buds infected the previous year - hard for big growers to do

ascocarps (chasmothecia)

form by midsummer

when buds are infected

no crop the following season

secondary infection

occurs when conidia from primary infections land on young, susceptible tissue

characterized by

patches of white to gray, powdery growth on young and older plant tissues - entire tissue surfaces covered with powdery growth

chasmothecia produced by secondary infections

play little role in disease development

significance

prior to 1940s, mildew was a disease of nursery stock and of minor importance to the commercial apple industry - increased incidence attributed to the switch from sulfur fungicides to organic fungicides better at controlling scab

best management strategy

resistant cultivars - reduces inputs of resources (such as sprays) needed to control disease

infected buds

serve as overwintering sites and provide primary inoculum - at budbreak, fungus resumes development - buds serve to produce inoculum in the spring, but fail to produce fruit

organic fungicides

single-site compounds - carbon-based, chemistry-based and are newer, synthesized compounds

most commonly observed on

the upper surface of leaves - reduce photosynthesis, increase respiration and transpiration, impair growth, and reduce yields (seldom kills host)

heavily infected trees

weakened and invaded by secondary pathogens

is it widespread?

yes, so widespread that losses surpass those caused by any other single type of plant disease