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- Author or Editor: J.L. Maas x
Fruit rots are probably the most important diseases of small fruit crops. Current statistics are not available for all small fruit crops, however, losses of strawberry fruit to pre- and postharvest fruit rots may conservatively be in the millions of dollars annually to growers, processors, and retailers. Losses from fruit rots vary from year to year and from locality to locality according to weather conditions prevailing during harvest. During wet weather, 30-50% or more of a strawberry crop may be lost to Botrytis rot, even with liberal use of fungicides. This, coupled with an increasing concern with development of fungicide-resistant strains of rot-causing organisms, particularly Botrytis cinerea Pers. ex Fr., and the development of mechanical harvesting methods have intensified the need to identify small fruit germplasm sources for fruit rot resistance.
Bacterial angular leafspot disease (BALD) of strawberry, caused by Xanthomonas fragariae, a slow-growing and often difficult pathogen to isolate from infected plants, is most commonly manifested as small discrete, angular, translucent lesions on leaves and sepals. As the bacteria infect systemically, plants may wilt and die. BALD has become increasingly important in North America and other strawberry-growing areas of the world. The systemic nature of the pathogen also is cause for concern with international shipment of strawberry plants, especially because there is no practical method for determining the presence of the bacteria in symptomless, infected plants, nor is there a practical method of chemical control. All cultivars of Fragaria × ananassa (8×) are susceptible to BALD, although a range of susceptibility is often apparent in plantings. Resistant genotypes have been reported among clones of F. virginiana (8×), F. moschata (6×), and F. vesca (2×). A program has been initiated to evaluate native octoploid and diploid strawberry germplasm for resistance to BALD.
We report the use of nuclear magnetic resonance (NMR) imaging to detect differences in invasion and colonization of fruit by pathogens (Botrytis cinerea, Colletotrichum acutatum, and Phytophthora cactorum), and bruise wounds are sharply distinguishable from healthy fruit tissue by their T1 times. Digitized images from T1 images clearly show two or more zones of pathogen activity in fruit tissue. The innermost zone corresponds to the area of greatest invasive activity at the leading margin of the infection. A second zone corresponds to the area of tissue that has been killed and is being degraded by the pathogen. Sometimes, a third zone is present at the outer border of the lesion and this correspond to where aerial sporulation may occur. Images of bruises, however, are uniform with no apparent gradations in T1 characteristics. Detection of fruit deterioration and decay is important in understanding and controlling postharvest loss of fruit crops. The nondestructive nature of MRI provides a means to quantify the process of decay development and control measures applied to fruits.
Nuclear magnetic resonance (NMR) can be used to examine tissue structure and developmental changes during growth and maturation of plant organs nondestructively. Spin-lattice, relaxation time (T1)-weighted, inversion recovery, spin-echo images of strawberry (Fragaria×ananassa Duch.) flower buds were acquired at 3 and 1 day before anthesis and receptacles at 4, 10, 15, and 25 days after anthesis (DAA). The central pith and ovules of flower buds imaged intensely with inversion echo times between 0.1 and 0.5 seconds. Achenes and the vascular cylinder, composed of vascular bundles surrounding the pith, were prominent in receptacles at 4 and 10 DAA. Vascular bundles leading to achene positions, cortex and pith tissues, and the vascular cylinder were evident in receptacles at all developmental stages. A general trend to homogeneity of structure was observed in images of receptacles nearing full maturity (25 DAA). Inversion recovery, spin-echo NMR microimaging may be useful for studying internal physicochemical changes in flower buds and fruit of strawberry and of other fruit crops.
Fruit of ‘Earliglow’ (Fragaria ⨯ ananassa Duch.) showed a significantly low incidence of postharvest rot caused primarily by Botrytis cinerea Pers. ex Fr. in 2 years of testing. Incidence of rot was modified by environmental factors during postharvest storage, particularly by cold storage.
The cultivars Allstar, Honeoye, Kent, and Jewel yielded the most successful summer and subsequent spring crops when planted in mid-July from dormant, cold-stored, multiple-crowned, nursery mother plants. Summer fruit sizing during very high temperatures was a problem; fruit number and quality was not. In a second hot year, the same cultivars did not give satisfactory summer crops when planted after late July. The return spring crop was most successful following planting in July rather than August or September, though there was a significant cultivar × planting date interaction for a number of characters. Nursery mother plants were more productive than first daughter plants. `Tristar' outyielded `Seascape' on black poly-mulched beds but not on killed vetch sod beds. `Allstar' runnered freely and produced good crops on three types of raised bed killed sod mulches and on red, blue, and silver-painted black poly raised bed mulches. Compared to the summer-planted, black, poly-mulched standard, `Mohawk' had increased but later yields, when the poly was painted blue, red, or silver, and much later yields, when mulched with recycled paper or wood fiber. The silver and paper treatments depressed fruit size. The wood fiber mulch seems promising for deliberately delaying ripening by lowering soil temperatures under the mulch. Seedling and selection plantings have generally responded favorably to summer planting from potted or “plug” plant stocks on unfumigated soil, thus, shortening the selection and evaluation cycles, with accompanying savings in land, water, fertilizer, and pesticide use.
‘Lester’ strawberry (Fragaria × ananassa Duchesne) was bred and selected to overcome a serious fault (red stele root rot susceptibility) of its productive, high-quality ‘Raritan’ parent, a major cultivar in the northeastern United States. ‘Lester’ offers strawberry growers of this region a cultivar that is resistant to red stele root rot, has attractive, good quality fruit, and ripens 2 to 3 days before ‘Raritan’. The cultivar was named for Lester W. Greeley of Farmington, N.H., who recently retired from the USDA, where he was associated with small grain, vegetable, and small fruit improvement programs for a period spanning more than 30 years. Greeley first noted the consistent plant and fruit production and high fruit recovery (often >90% marketable) of the selection later named in his honor.
Four methods of inoculation with Verticillium were tested for effectiveness in infecting strawberry plants grown in a greenhouse bench. The most severe and early symptoms were produced with a macerated mycelium root dip inoculum. Effect of inoculum aggressiveness on the extrapolation of plant resistance information is discussed.
We have determined in tests conducted both at Beltsville and Poplarville that several strawberry isolates of Colletotrichum acutatum, C. gloeosporioides and C. fragariae produce toxin-like compounds in culture. Crude culture filtrates (CFI elicited general and specific responses in tomato and strawberry plants. Tomato plants initially were used because they are highly responsive to toxins in general, whereas the reaction of strawberry plants apparently is greatly affected by environmental and nutritional growing conditions of the test plant. Toxin symptoms included leaf chlorosis and wilting, leaf midvein darkening, and plant death when CF was applied to leaves or if seedlings or petioles were immersed into CF. Juvenile tissues appear to be more susceptible to the effects of the toxins than mature tissue. No differences in response to culture filtrates were apparent among those from the Colletotrichum isolates. The putative toxins appear to act differentially with susceptible or resistant strawberry germplasm.