Field and glasshouse observations of Lolium spp. grasses indicated that the lower, abaxial, leaf surface was rarely infected by powdery mildew (Erysiphe graminis) even when the upper, adaxial, surface was densely colonized. Experiments showed that conidia of two strains of E. graminis, one from Lolium and one from Avena, germinated equally well on both surfaces of Lolium and Avena leaves, but that the subsequent growth and development of germlings was impaired on the lower surface of Lolium leaves, so that most formed only multiple short germ tubes or an abnormal long tube, and only c. 25% or fewer formed infection structures. This contributes to the apparent resistance of the lower Lolium leaf surface to powdery mildew and may help to explain why the disease is relatively unimportant in UK ryegrass crops, since infection structures develop at a high frequency on only 50% of the leaf area, i.e. the upper surface. Scanning electron microscopy showed that the epicuticular waxes on the lower Lolium leaf surface form amorphous sheets. This contrasts with the crystalline plate waxes seen on the upper surface of Lolium leaves and on both surfaces of oat leaves. However, when the lower Lolium leaf surface was washed with chloroform to remove epicuticular wax, normal germling and infection structure development was obtained on the wax-free surface. This suggests that the sheet waxes prevent the pathogen gaining access to features of the cuticular membrane which trigger normal germling development.

The role of the abaxial leaf surface waxes of Lolium spp in resistance to Erysiphe graminis.

Plant surface characteristics were repeatedly shown to play a pivotal role in plant–pathogen interactions. The abaxial leaf surface of perennial ryegrass (Lolium perenne) is extremely glossy and wettable compared to the glaucous and more hydrophobic adaxial surface. Earlier investigations have demonstrated that the abaxial leaf surface was rarely infected by powdery mildew (Blumeria graminis), even when the adaxial surface was densely colonized. This led to the assumption that components of the abaxial epicuticular leaf wax might contribute to the observed impairment of growth and development of B. graminis conidia on abaxial surfaces of L. perenne. To re-assess this hypothesis, we analyzed abundance and chemical composition of L. perenne ab- and adaxial epicuticular wax fractions. While the adaxial epicuticular waxes were dominated by primary alcohols and esters, the abaxial fraction was mainly composed of n-alkanes and aldehydes. However, the major germination and differentiation inducing compound, the C26-aldehyde n-hexacosanal, was not present in the abaxial epicuticular waxes. Spiking of isolated abaxial epicuticular Lolium waxes with synthetically produced n-hexacosanal allowed reconstituting germination and differentiation rates of B. graminis in an in vitro germination assay using wax-coated glass slides. Hence, the absence of the C26-aldehyde from the abaxial surface in combination with a distinctly reduced surface hydrophobicity appears to be primarily responsible for the failure of normal germling development of B. graminis on the abaxial leaf surfaces of L. perenne.

Two sides of a leaf blade: Blumeria graminis needs chemical cues in cuticular waxes of Lolium perenne for germination and differentiation

Publisher Summary This chapter discusses the water and quality loss during field drying of hay The objective of haymaking is to conserve the yield and nutritional value of fresh-cut forage by drying it as quickly as possible to a level at which the activity of microbial decomposers is halted The moisture content theoretically required to prevent microbial activity is 10–12%, but in practice, dry hay is baled and stored at from 15– 20% moisture, which incurs low but acceptable levels of microbial damage Rate of water loss during field drying of cut hay typically declines exponentially, such that each additional percentage drop in moisture content requires progressively more time Heavy, high-moisture hay, particularly if composed of juvenile, leafy tissues rather than stiff, reproductive stems, tends to compact or settle under its own weight, reducing air circulation within the windrow, and increasing boundary layer resistance Predictive models of forage drying have identified irradiance as having the greatest influence on both alfalfa and grass drying rate, with other significant contributions from the drying potential of the environment, expressed as potential evapotranspiration or vapor pressure deficit, soil moisture, and windrow density It is found that practical methods of facilitating water loss from cut hay include growing grasses and legumes in mixture to lessen the impact of the slower drying legumes

D.M. Power

Papers

The chemical composition of the surface wax of lolium perenne