Sporulation potential, symptom expression and detection of Phytophthora ramorum on larch needles and other foliar hosts

Abstract

Phytophthora 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 that then cause bark lesions and girdling cankers on trees. Following inoculation, individual needles of Japanese, European (L. decidua) and hybrid (L. 9 eurolepis) larch infected with P. ramorum can produce thousands of sporangia. Mean numbers of sporangia ranged from 806 to 1778 per cm 2 (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, without any sign of discolouration or necrosis. Plating infected but symptomless needles onto Phytophthora selective medium also often failed to yield the pathogen. Symptomless infection of larch needles apparently occurs, but is only detectable with microscopy. More generally, it is suggested that diagnosis of Phytophthora infection in conifers is often underestimated due to isolation difficulties and delayed symptom expression.

Assessment of the eradication measures applied to Phytophthora ramorum in Irish Larix kaempferi forests

“…() that baited 23 Phytophthora taxa from soil samples in European coniferous forests and even from our previous work in a range of habitats across Ireland (O'Hanlon et al., ). The isolation success rate for P. ramorum from L. kaempferi samples is very low (Harris & Webber, ), and it is possible that the soil and leaf litter in L. kaempferi ‐associated sites has a suppressive effect on P. ramorum , as has been found in redwood forests in California (Fichtner, Lynch, & Rizzo, ). The lack of positive findings from soil samples was mirrored in the lack of positive findings from the footwash samples take in this study.…”

“…Early detection and rapid eradication of infected sites are vital to containing the P. ramorum epidemic on Larix. The long-distance dispersal capability of P. ramorum (Peterson et al, 2015), its ability to asymptomatically infect L. kaempferi (Harris & Webber, 2016), and the causes of the difficulties in isolating P. ramorum cultures from L. kaempferi material (Harris, 2014) are areas where future research is needed to increase the effectiveness of the eradication and control efforts in Ireland and the UK.…”

“…5% (5,500 ha; McCracken et al., ) of the tree species composition of the Irish and Northern Irish forest estates, respectively. These hosts are regularly found infected in Ireland (O'Hanlon et al., ) and are known to support high levels of P. ramorum sporulation (Harris & Webber, ). Other species of Larix , such as hybrid larch ( L .…”

“…Other species of Larix , such as hybrid larch ( L . × marschlinsii ) and European larch ( Larix decidua ), can also apparently support high levels of sporulation (Harris & Webber, ) but are likely to be less important hosts in the sudden larch death epidemic as they have only a limited distribution across Ireland (NFI ).…”

“…5% (5,500 ha;McCracken et al, 2015) of the tree species composition of the Irish and Northern Irish forest estates, respectively. These hosts are regularly found infected in Ireland (O'Hanlon et al, 2016) and are known to support high levels of P. ramorum sporulation (Harris & Webber, 2016). Other species of Larix, such as hybrid larch (L. × marschlinsii) and European larch (Larix decidua), can also apparently support high levels of sporulation (Harris & Webber, 2016) but are likely to be less important hosts in the sudden larch death epidemic as they have only a limited distribution across Ireland (NFI 2012).…”