A total of 1789 accessions of several lettuce collections was screened to find new major gene resistance to the downy mildew fungus Bremia lactucae Regel . The accessions belonged to the species Lactuca sativa (N = 1288), L . serriola (N = 399), L . saligna (N = 52) and L . virosa (N = 50) . A total of 20 races of B . lactucae were used, 14 of which were NL-races, isolated from cultivated lettuce in the Netherlands . The other six races were isolated from wild L . serriola in Czechoslovakia . The accessions were initially screened with two races : NL1 and NL3 . Accessions with resistance to one or both of these races were tested with the other races . Phenotypes with new resistance were found in accessions of all four Lactuca species . Of L . sativa, four accessions were found with resistance phenotypes that could not be explained by combinations of known major genes . Many accessions of L . serriola had resistance phenotypes that indicated the presence of unknown resistance genes . All interactions between accessions of L . saligna and races of B . lactucae were incompatible in leaf disc tests, except for four accessions, which showed some sporulation with race NL6 . Several accessions of L . virosa were resistant to all races used . Other accessions of L . virosa gave a race-specific interaction with B . lactucae .
The viability of `Avignon' and `Esther' (Asiatic hybrids), `Star Gazer' (Oriental hybrid), and `Snow Queen' lily bulb scales (L. longiflorum Thunb.) was determined after storage at –2C for 0.5, 1.5, and 2.5 years. Ion leakage, the percentage of scales that formed bulblets, and the number and weight of these bulblets were determined on scales from the inner, middle, and outer part of bulbs. During storage, the outer scales of all cultivars and the inner and middle scales of `Snow Queen' showed increased ion leakage accompanied by a decreased ability to form scale bulblets during storage. Concurrently, the percentage of scales forming bulblets declined, and more and smaller scale bulblets were formed per regenerative scale. Thus, ion leakage is a useful criterion to measure viability of lily scales.
Sugar-beet plants, infected with beet yellows virus (BYV, closterovirus group) or beet mild yellowing virus (BMYV, luteovirus group) develop symptoms on the inoculated leaves on which aphids infected the plant. Symptoms develop also on the systemically-infected leaves to which virus has been transported via the phloem. Systemic infection occurs in the leaves which have just, or not yet appeared at the moment of infection of the plant. All other, older leaves remain uninfected. The infection-date can be estimated by assessing the date of appearance of the oldest systemicallyinfected leaf of a plant. This approach was tested in the field and gave good results.Additional keywords: beet yellows virus, closterovirus, beet mild yellowing virus, luteovirus, systemic virus transport, phloem translocation, phyllotaxis, leaf arrangement, leaf appearance, temperature sum, symptom development Neth. J. Pl. Path. 95 (1989)
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