The Microbial Community of Grape Berry - Part 3

In the first blog, The Microbial Community of Grape Berry, the topic was about the four groups of microorganisms that include (1) residents, (2) adventitious, (3) invaders, and (4) opportunists that can populate the grape berry.
The second blog The Microbial Community of Grape Berry - Part 2 was about the grape berry cuticle and the important role it plays in protecting the grape berry.
In today's blog we bring both topics together and ask, when are these four groups of microorganisms on the grape berry and what impact does it have on the condition of the berry?
The answer to this question depends fundamentally on:

• the berry development stage
• the intactness of the grape skin
• the prevailing environmental conditions

The Berry Development Stage
Grape berry development consists of two distinct but successive sigmoidal growth periods separated by a lag phase.
The first, the berry formation stage, also termed herbaceous or green phase, is the first period of growth, lasting from bloom to approximately 60 days afterwards.
Veraison or the ripening stage occurs 8-14 weeks after flowering and begins the second growth phase typified by sugar accumulation, skin softening , and skin coloration in the case of red varieties. The second period of growth is to make the berry as appealing as possible to birds and mammals so that seed dispersal may occur.
The following diagram clearly shows the berry development stage and the microorganisms that are prevalent at that stage.¹

• resident microorganisms are present on healthy grape surfaces at all times during their development
• invader infections occur essentially during the berry formation stage
• opportunist infections are concentrated during the berry ripening phase

Intact Berries

• in intact berries, the residents will be most prevalent.
• the residents can be rapidly superseded by the invaders or the opportunists when they proliferate through a berry wound, either of abiotic (i.e. rainfall, wind, temperature, viticultural practices, etc.) or biotic (i.e., diseases, pests, an insect or a larger animal) origin that can affect skin integrity.
• loss of skin integrity paves the way for massive growth of opportunists adapted to the low pH (3.0 to 3.8) and high sugar concentration (ca. 200 g.L-1) of grape juice
• the wound largely influences the microbiota which subsequently colonizes the damaged grape since it will induce a rapid change in the blend of grapevine volatile organic compounds (VOCs) emitted.
• G. Oxydans, predominates in sound berries, but is superseded by Gluconoacetobacter and Acetobacter, namely A. aceti and A. pasteurianus, in rotten berries.
• B. cinerea, the causal agent of grey rot, responsible for intense damage when rainfall is abundant, especially towards the end of grape ripening, as a result of berry splitting
• damaged grapes possess much higher cell counts (up to 5 log cycles) and display wider species diversity than sound grapes

Prevailing Environmental Conditions

• Downy Mildew
  →when warm and wet weather conditions prevail during the vegetative growth of the vine, downy mildew caused by the oomycete, obligate biotroph Plasmopara viticola is one of the most widespread and destructive diseases of grapevine
  →spring and summer rainfall at temperatures above 50ºF releases the oospores which is the primary inoculum
  →the occurrence, development and spread of disease are mainly dependent on rainfall and temperature
  → oospore, the primary source of infection, germinates on moist soil or in the presence of free water when temperatures rise above 11 ^oC, producing sporangia
  
• Powdery Mildew
  →Erysiphe necator is also known as Uncinula necator and is commonly called oidium
  →prefers mild temperatures between 68 and 85°F, relative humidity levels above 75%, and low levels of solar radiation
  
• Botrytis cinerea
  →under cool, wet and humid conditions, Botrytis bunch rot which favour sporulation and infection occurs
  →the severity of the infection by the fungus requires at least 25 h at 15-20 °C and saturated relative humidity
  →berry wounds constitute a preferential way for B. cinerea entry since it is an opportunist
  →susceptibility of grapes to this infection has been shown to greatly increase at the onset of grape ripening (veraison)
  →high free water level, suitable temperature and the presence of nutrients on the skin surface are necessary to induce spore germination and fungal growth
  

References:
1. V. Loureiro, M. M. Ferreira1, S. Monteiro1 and R. B. Ferreira, "The Microbial Community of Grape Berry", The Biochemistry of the Grape Berry, Chapter 12, 2012, 241-268.