Sporulation Potential, Symptom Expression and Detection of Phytophthora ramorum on Larch Needles and Other Foliar Hosts

Author ,
Date In press
Publication Plant Pathology
Key Words
DOI10.1111/ppa.12538.
AbstractPhytophthora ramorum has caused extensive dieback and mortality of commercially grown Japanese larch (Larix kaempferi) in many parts of the UK as infected foliage generates spores which then cause bark lesions and girdling cankers on trees. Following inoculation individual needles of Japanese, European (L. decidua) and hybrid (L. x eurolepis) larch infected with P. ramorum could produce thousands of sporangia. Mean numbers of sporangia ranged from 806 to 1,778 per cm2 (hybrid larch and Japanese larch respectively), surpassing mean sporulation levels on foliar hosts previously associated with P. ramorum outbreaks in Britain, namely Rhododendron ponticum, Castanea sativa and Vaccinium myrtillus. Sporulation on larch even exceeded that of California bay laurel (Umbellularia californica) which drives the Sudden Oak Death epidemic in California. Inoculation of foliage selected at different times of year revealed that foliage age significantly affected sporulation levels, but this varied with host species. However, symptom development and sporulation were often not correlated. Symptoms on larch were frequently insignificant or even absent at certain times of year, with sometimes the only evidence of infection being the emergence of sporangia from needles but without any sign of discolouration or necrosis. Plating infected but asymptomatic needles onto Phytophthora selective medium also often failed to yield the pathogen. Asymptomatic infection of larch needles apparently occurs but is only detectable with microscopy. More generally, we suggest that diagnosis of Phytophthora infection in conifers is often underestimated due to isolation difficulties and delayed symptom expression.
Full Citation Harris, A.R. and Webber, J.F. In press. Sporulation Potential, Symptom Expression and Detection of Phytophthora ramorum on Larch Needles and Other Foliar Hosts. Plant Pathology. DOI: 10.1111/ppa.12538.