Fragariae Var Research Articles

A synopsis of the occurrence and pathogenicity of Phytophthora species in mainland China.

To date, 26 species of Phytophthora have been reported in mainland China but only 23 are accepted as good taxa. Phytophthora nicotianae (P. parasitica) is the single most important species causing over 40 different plant diseases; P. boehmeriae is widespread attacking Gossypium, Boehmeria, Citrus, Broussonetia papyrifera and Pterocarya stenoptera; P. "fragariae var. oryzo-bladis" is unique to mainland China causing blight of Oryza sativa seedlings; P. megasperma sensu lato has been isolated from Glycine max, Asparagus officinalis and Spinacia oleracea. In Hainan Province, P. heveae is present in the soil without causing apparent diseases to Hevea brasiliensis there.

Aromatic Polyketide Synthases (Purification, Characterization, and Antibody Development to Benzalacetone Synthase from Raspberry Fruits).

p-Hydroxyphenylbutan-2-one, the characteristic aroma compound of raspberries (Rubus idaeus L.), is synthesized from p-coumaryl-coenzyme A and malonyl-coenzyme A in a two-step reaction sequence that is catalyzed by benzalacetone synthase and benzalacetone reductase (W. Borejsza-Wysocki and G. Hrazdina [1994] Phytochemistry 35: 623-628). Benzalacetone synthase condenses one malonate with p-coumarate to form the pathway intermediate p-hydroxyphenylbut-3-ene-2-one (p-hydroxybenzalacetone) in a reaction that is similar to those catalyzed by chalcone and stilbene synthases. We have obtained an enzyme preparation from ripe raspberries that was preferentially enriched in benzalacetone synthase (approximately 170-fold) over chalcone synthase (approximately 14-fold) activity. This preparation was used to characterize benzalacetone synthase and to develop polyclonal antibodies in rabbits. Benzalacetone synthase showed similarity in its molecular properties to chalcone synthase but differed distinctly in its substrate specificity, response to 2-mercaptoethanol and ethylene glycol, and induction in cell-suspension cultures. The product of the enzyme, p-hydroxybenzalacetone, inhibited mycelial growth of the raspberry pathogen Phytophthora fragariae var rubi at 250 [mu]M. We do not know whether the dual activity in the benzalacetone synthase preparation is the result of a bifunctional enzyme or is caused by contamination with chalcone synthase that was also present. The rapid induction of the enzyme in cell-suspension cultures upon addition of yeast extract and the toxicity of its product, p-hydroxybenzalacetone, to phytopathogenic fungi also suggest that the pathway may be part of a plant defense response.

Effect of temperature and host genotype on the production of inoculum by Phytophthora fragariae var. fragariae from the roots of infected strawberry plants

A bioassay was used to monitor the release of inoculum in drainage water from strawberry plants inoculated with zoospores of Phytophthora fragariae var. fragariae. The fungus was detected in drainage water from plants that had been held at temperatures between 2 and 20 C. but not from plants held at 26°C. The lag phase before secondary inoculum was first released, the maximum and total amounts of inoculum released, and the length of time over which inoculum was released were all greater at the lower temperature regimes, especially those below 10 C. The results were consistent with observations on the effect of temperature on zoospore production from agar discs and on zoospore motility: more zoospores were produced at lower temperatures and they remained motile for longer. From this it is concluded that the inoculum detected consists mainly of motile zoospores. In most experiments with standardized suspensions c. 10‐15 were sufficient to initiate infection of the plants in the bioassay. In general, more inoculum was produced by host genotype/fungal isolate combinations in which there were marked root rot symptoms than in combinations in which the host was resistant.

Control of root rot of red raspberries caused by Phytophthora fragariae var. rubi

The effects of raised beds, mulching, organic amendments and soil treatment with metalaxyl on raspberry root rot (Phytophthora fragariae var. rubi) were studied in a field experiment with two raspberry cultivars, Veten (highly susceptible) and Chilliwack, a less susceptible cultivar. Raised beds reduced the severity of root rot compared with flat beds. The best results for both cultivars were obtained with a combination of raised beds and metalaxyl treatment. As separate treatments, raised beds and metalaxyl had approximately the same effect. Mulching with grass or composted manure increased the severity of root rot.

Genetic variation for resistance to Phytophthora fragariae var. rubi in red raspberries

The inheritance of root rot resistance in red raspberry, caused by Phytophtora fragariae var. rubi, was investigated using two experimental breeding designs. Design A was not randomly mated and genetic parameters could not be calculated. Design B was therefore randomly mated to make it possible to do such estimations. Considerable genetic variation was associated with the recorded traits on seedlings tested in a glasshouse. Disease index and % survival were both found to be highly heritable with broad sense heritabilities (H2) close to 0.9 in each design. Estimates of narrow sense heritability (h2), indicated that progress towards resistance would be moderate. In the first design the cultivars ‘Festival’, ‘Asker’, ‘Boyne’ and ‘Sentry’ and the clones N85-02-13 and N85-06-140 had high general combining abilities. Some crosses had a high specific combining ability indicating the presence of non-additive genetic variation.

012 EXPLOITATION OF RUBUS STRIGOSUS IN RASPBERRY BREEDING

The indigenous North American red raspberry, Rubis strigosus has been neglected in breeding programs. Only four cultivars, `Cuthbert', `Latham', `Herbert' and `Ranere' provide most of the germplasm contained in present-day cultivars; no more than six individual wild genotypes of the species are represented by the four cultivars. In recent years, the B.C. breeding program has screened seedling populations of hitherto unexploited genotypes of the species from various locations in North America. Useful traits identified in selections from the populations include levels of resistance to 1) the North American aphid vector, Amphorophora agathonica, of the raspberry mosaic virus complex, 2) to several cane diseases and 3) to root rot caused by Phytophthora fragariae var rubi, as well as desirable fruit traits, such as bright, non-darkening red color and easy release. Selections with cultivar potential have now been identified in the second and third backcross generations from the species.

Analysis of RFLPs in nuclear and mitochondrial DNA and the taxonomy of Phytophthora fragariae

Severe root rot of red raspberry is caused by a fungus which has recently been classified as P. fragariae var. rubi because it is thought to be closely related to the fungus causing red core disease of strawberry, P. fragariae var. fragariae. DNA from P. fragariae var. rubi was cloned in E. coli, and clones showing little or no hybridization to other Phytophthora species in dot blots were selected for use as probes which might detect restriction fragment-length polymorphisms (RFLPs) within and between the so-called varieties. Variation revealed by restriction enzyme digestion of mtDNA was also employed. Both pathogens were very homogeneous, providing support for their recent and rapid worldwide expansion. Although readily distinguishable, they were more closely related to one another than to any other Phytophthora spp.; their separation at the variety level appears appropriate. Two of the probes also proved suitable for differentiating among other Phytophthora spp. associated with raspberry root rot.