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L. Eric Hinesley, K.C. Parker, and D.M. Benson

Containerized seedlings of Fraser fir [Abies fraseri (Pursh) Poir.], momi fir (A. Firma Sieb. & Zucc.), and Siberian fir (A. siberica Ledeb. var. argentea) were tested for resistance to Phytophthora cinnamomi Rands (two experiments). Fraser fir was very susceptible, with many plants dead after 4 to 5 weeks. Symptoms developed more slowly in Siberian fir, but most plants were dead or dying after 10 weeks. Momi fir appeared somewhat resistant to the pathogen. Though undesirable as a Christmas tree, momi fir might be useful as a rootstock for grafting better quality species in areas affected by root rot.

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S.L. Krebs

Root rot caused by the soil-borne pathogen Phytophthora cinnamomi is one of the deadliest and most costly diseases in rhododendron culture. Unfortunately, the majority of cultivars appear to be susceptible to this fungus. Host resistance does occur, but it represents a tolerance of rather than immunity from the disease. A breeding program has been initiated to develop a broader array of root rot resistant cultivars and to determine the genetic basis for resistance. Greenhouse inoculations and screenings of 48 contemporary cultivars yielded seven clones with moderate to high levels of resistance to P. cinnamomi. Protocols for evaluation at the seedling stage were developed in order to screen large breeding populations of about 200 seedlings per cross. Root rot tolerance appears to have low-moderate heritability in these rhododendron populations. Groups of progeny with one resistant parent had a slower mortality rate and higher survivorship (avg. 10%) after 2 months of disease pressure than crosses in which both parents were susceptible (0 survivorship). A recurrent selection strategy is planned to increase the frequency of alleles for resistance in breeding populations of rhododendrons.

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Katherine B. Wing, Marvin P. Pritts, and Wayne F. Wilcox

Blackening and decay of roots in association with plant stunting are common in perennial strawberry (Fragaria ×ananassa Duch.) plantings worldwide; this syndrome is commonly referred to as black root rot (BRR), although its causal agent(s) are not well characterized. We conducted a New York field survey that measured many physical and cultural factors in healthy and diseased fields to identify those most strongly associated with BRR. Factors significantly correlated with BRR symptoms were soil compaction, fine soil texture, absence of raised beds, high application rates of the herbicide terbacil, advanced age of planting, nonuse of the fungicide metalaxyl, and cumulative years of strawberry monoculture. Populations of Pratylenchus spp. were not associated with poor root health. The data suggest that most factors that compromise root growth may predispose strawberry plants to infection by site-specific BRR pathogens. Chemical names used: 3-tert-butyl-5-chloro-6-methyluracil (terbacil); N-(2,6-dimethylphenyl)-N-(methoxyacetyl) alanine methyl ester (metalaxyl).

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James F. Hancock, Peter W. Callow, Sedat Serçe, and Annemiek C. Schilder

1 To whom requests for reprints should be addressed. E-mail address: hancock@pilot.msu.edu We thank Fred Warner for nematode counts, John Rogers for pathogen identification and Jeri Gillett for general helpful

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B.K. Duffy and G. Défago

Host nutritional variables were evaluated for their effects on the severity of crown and root rot of tomato caused by Fusarium oxysporum f.sp. radicis-lycopersici. Tomato (Lycopersicon esculentum Mill.) seedlings (cv. Bonnie Best) were grown in a pathogen-infested, soilless rockwool system in the greenhouse and were fertilized with a nutrient solution that was amended with macro- and microelements at various rates. Disease was evaluated after 2 weeks using an index of 0 to 4, and plant fresh weight was measured. Regression analysis indicated that disease severity was significantly increased by ammonium-nitrogen [NH4Cl, (NH4)6Mo7O24, and (NH4)2SO4], NaH2PO4·H2O, Fe-EDDHA, MnSO4, MoO3, and ZnSO4·7H2O. Disease severity was reduced by nitrate-nitrogen [Ca(NO3)2·4H2O] and CuSO4·H2O. Low rates of NH4NO3 (39 to 79 mg·L-1 N) reduced disease, but rates above 100 mg·L-1 N increased it. Disease was not affected by MgSO4·7H2O. In all cases, plant growth was inversely related to disease severity. Mineral fertilizers had no effect on nutrient solution pH. This information sheds new light on environmental factors that influence plant-pathogen interactions, and may be applied to develop a management strategy for Fusarium crown and root rot based on host nutrition.

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W. Alan Erb and Randall C. Rowe

Two procedures for screening tomato (Lycopersicon esculentum Mill.) seedlings for resistance to three pathogens were developed. In one scheme, seeds were sprayed with a spore suspension of Fusarium oxysporum f. sp. radicis-lycopersici Jarvis & Shoemaker (fusarium crown and root rot). Resistant seedlings were root-dipped 2.5 weeks later in a spore suspension of Verticillium dahliae Kleb. (verticillium wilt), and 1 week following the root dip, leaves were rubbed with tobacco mosaic virus. In the other scheme, 2-week-old seedlings were dipped in a spore suspension of F. oxysporum Schlecht f. sp. lycopersici (Sacc.) Snyd. & Hans. races 1 and 2 (fusarium wilt). Resistant seedlings were root-drenched 1.5 weeks later with a suspension of Meloidogyne incognita Kofoid & White (rootknot nematode), and 1 week following, the leaves were rubbed with tobacco mosaic virus. These procedures were effective for disease screening, and their use should reduce the time required for development of two multiple disease-resistant populations. Inbreds from each population could be crossed to produce hybrids resistant to five pathogens.

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Monte L. Nesbitt, J.B. Storey, S.D. Lyda, and L.J. Grauke

Phymatotrichum Root Rot, caused by Phymatotrichum omnivorum (Shear) Duggar, imposes severe losses upon dicotyledonous horticultural crops in the southwestern United States and northern Mexico. Rootstock resistance could benefit pecan (Carya illinoensis) production in affected growing areas; however, erratic growth habit of this pathogen and site variability prevents effective field screening. We have developed a containerized screening method for horticultural crops, using a commercial soilless growing medium. In sterile cultures, 2.5 × 60 cm glass tubes containing Metro Mix 500 yielded more grams of P. omnivorum sclerotia than cultures grown in Houston Black Clay, a traditional medium for cotton research. Preliminary screening with Okra (Abelmoschus esculentus) in non-sterile Metro-Mix 500 resulted in 75% mortality of inoculated plants in 30-35 days. Preliminary screening with 12-week-old, open-pollinated `Apache' and `Moore' pecan rootstocks has resulted in 25% mortality of inoculated plants in 150 days. Pecan seedlings with visible taproot infection appear to delay mortality by adventitious root formation.

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Jim Downer, Ben Faber, and John Menge

Mulches can exert positive (disease controlling) or negative (disease enhancing) potential when applied to young avocado (Persea americana) trees. Regulation of root disease in avocado is a complicated process that is affected by host resistance, inoculum density, temperature, soil salinity and soil water potential. There are short-term immediate effects from mulching and subtle long-term effects that regulate disease caused by the root rot pathogen Phytophthora cinnamomi. Short-term effects include increased soil moisture and soil temperature moderation. Long-term effects include increases of: soil mineral nutrients, soil aggregation and drainage; microbial activity; and cellulase enzyme activities. Biological control of Phytophthora in mulched soil is partially regulated by cellulase enzyme activities. This soil enzyme concept of biological control is discussed in regard to the classical Ashburner method of biological control.

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Ousmane Sy and Paul W. Bosland

The oomycete fungus Phytophthora capsici Leon. is known to be a limiting factor of chile pepper (Capsicum spp.) production around the world. The genetics of the resistance is becoming better understood due to the specific nature of the host–pathogen interaction; i.e., all plant organs are subject to infection. This study determined whether stem blight was the same disease syndrome as root rot or foliar blight. Stem cuttings of a segregating F2 population and testcross progeny facilitated the ability to screen for two disease syndromes concurrently. When the three disease syndromes were compared separately, the F2 populations fit a 3R:1S ratio and the testcross progeny fit a 1R:1S ratio. When comparative studies were performed (stem vs. foliar and stem vs. root), the F2 populations fit a 9R/R:3R/S:3S/R:1S/S ratio and the testcross fit a 1R/R:1R/S:1S/R:1S/S ratio. These ratios are consistent of a single gene controlling the resistance of each system. Therefore, Phytophthora stem blight, root rot, and foliar blight are three separate disease syndromes.

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Todd L. Mervosh and James A. LaMondia

The effects of terbacil herbicide on strawberry (Fragaria ×ananassa Duch. `Honeoye') yield and black root rot disease were determined in field plots at two locations in Connecticut over 4 years. Terbacil treatments at up to four times the maximum label dosage caused some temporary foliar chlorosis but did not affect the health of structural or perennial roots and associated feeder roots. Development of secondary root growth (perennial roots) was not influenced by terbacil. Terbacil had no effect on the quantity of lesion nematodes [Pratylenchus penetrans (Cobb) Filip & Schur. Stek.] extracted or the amount of the fungal pathogen Rhizoctonia fragariae Husain and McKeen isolated from strawberry roots. At both locations, R. fragariae was common on plant roots by the fourth year. Terbacil treatments did not affect strawberry yields in terms of number or weight of ripe berries per plot. Our results indicate that terbacil does not contribute to black root rot or decreased yields in `Honeoye' strawberry. Chemical name used: 5-chloro-3-(1,1-dimethylethyl)-6-methyl-2,4-(1H,3H)-pyrimidinedione (terbacil).