Lepto Leaf Spot Resistance
Leptosphaerulina briosiana (Poll.) Graham and Luttrel Kenneth Leath
1 Resistant......No spots
2 Resistant .....Barely visible black pepper spots, l mm diam. or less
3 Susceptible ..Spots >lmm, with or without tan center, no halo
4 Susceptible ..Spots with tan center, halo
5 Susceptible ..Spots >3mm, with tan center, halo, spots coalesced, leaf withered
Values for resistant standards are totals of l's and 2's
DISTRIBUTION AND SEVERITY OF LEPTO LEAF SPOT
Lepto leaf spot, Leptosphaerulina briosiana (Poll.)
Graham & Luttrell
SOURCE OF INOCULUM AND SCIENTIST WITH EXPERTISE
Name ............K.T. Leath
CORRELATION TO FIELD REACTION
Good correlations to field; no exceptions reported.
No races are known.
CULTURE OPTIONS AND RANGE OF CONDITIONS
Light is critical for spore production. Fluorescent or near UV is as effective as natural daylight. Some glass petri dishes do not pass sufficient light below 340 nm wavelength for good sporulation. Cultures can be started either by placing pieces of agar containing fungal hyphae onto agar surface or by spreading a spore suspension, (prepared by scraping surface of mature culture in water), over agar plate surface. The latter is the quicker method, but if plates contain bacterial contaminants they will not be usable. When cultures are ready to use, ascospores discharged onto petri dish lid are visible. These can serve as contaminant free source for subsequent cultures.
PLANT GROWTH OPTIONS AND RANGE OF CONDITIONS
Vigorous plants are essential for expression of susceptible response. Light intensity after inoculation must exceed 1000 mole m-2sec-1. Supplemental light (metal halide or other) is necessary during winter at some locations to produce vigorous plants. Use of lightweight potting mix is best if plants are to be pulled during scoring.
INOCULATION CONDITIONS AND RANGE OF CONDITIONS
Temperatures from 15 to 25°C are probably usable. Light during infection not required. Lower temperatures slow infection but seldom cause failure; too high temperatures or drying leaf surfaces will result in failure.HELPFUL INFORMATION
Plants may be cut back at scoring and regrowth used for different disease evaluation. Isolation of fungus is usually done by direct transfer of spores from sporulating pycnidia produced on leaf tissue. Infection does not kill stems or plants.
Field evaluations may be possible but field infections are rarely of sufficient purity, uniformity, and severity to facilitate satisfactory selection. Inoculations have been made by spraying spores onto leaves. Cultures are scraped in water, comminuted, and filtered through cheesecloth to remove large particles. This method often results in a light inoculation.
1. Leath, K.T. 1971. Quality of light required for sporulation by Leptosphaerulina. Phytopathology 61:70-72.
2. Leath, K.T., and R.R. Hill, Jr. 1974. Large incubation chamber suited for use in selection for disease resistance. Crop Sci. 14:901-903.
3. Leath, K.T., and R.R. Hill, Jr. 1974. Leptosphaerulina briosiana on alfalfa: relation of lesion size to leaf age and light intensity. Phytopathology 64:243-245.