Lettuce drop, caused by the soilborne fungi Sclerotinia minor and S. sclerotiorum, continues to be an important disease on this crop in Arizona. Trials were conducted over a 5-year period to compare different fungicides as well as the number, timing, and method of application. Compared with nontreated plots, disease reduction ranging from 49.6 to 61.0% was achieved on lettuce beds containing S. minor and treated with fluopyram + trifloxystrobin, fluazinam, fluxapyroxad + pyraclostrobin, and boscalid. Treatment of beds containing S. sclerotiorum with Coniothyrium minitans, iprodione, and boscalid reduced lettuce drop from 50.6 to 71.5%. No difference in disease control was noted between one and two applications of boscalid in plots containing either pathogen. In the presence of S. minor, beginning the first of two applications of boscalid after seeding did not differ from starting after thinning; however, in plots containing S. sclerotiorum, starting application after seeding was superior to beginning after thinning. Physical incorporation of soil treated with boscalid to a depth of 5.0 cm did not differ from soil treatment without incorporation in plots containing S. minor or S. sclerotiorum. On the other hand, in beds containing S. sclerotiorum, incorporation of soil treated with iprodione improved disease control compared with no incorporation.

A unique foliar disease of spinach, determined to be caused by Pythium aphanidermatum, was observed on spinach in Yuma County, AZ and Imperial County, CA desert spinach production areas in both 2015 and 2016. The foliar symptoms of the disease included water-soaked foliage, rapid collapse of young plants, and white, aerial, cottony mycelia. The disease was associated with hot (27 to 42°C) and wet conditions associated with overhead irrigation under high-density plantings (>8.0 million seeds/ha). Isolations were performed on symptomatic tissue, and DNA was recovered from pure culture of the isolates recovered and sequenced using the internal transcribed spacer (ITS) ribosomal DNA (rDNA) primers ITS1/ITS4 and gene cytochrome oxidase I (COXI) primers FM55 and FM59. BLAST searches in GenBank indicated that the isolates were P. aphanidermatum based on 99 to 100% homology of ITS rDNA. Moreover, the DNA sequences of the ITS and COXI were identical for the five representative isolates. The objective of this research was to determine whether P. aphanidermatum recovered from symptomatic spinach tissue was able to cause foliar web blight and damping-off of spinach and other crops. In addition to spinach, other hosts evaluated included cotton, soybean, pepper, tomato, cucumber, melon, squash, lettuce, corn, wheat, and rice in greenhouse trials. Inoculations were performed by either foliar inoculations or infesting the soil with plugs of potato dextrose agar colonized by the P. aphanidermatum. Web blight symptoms were severe on spinach and all other dicotyledonous hosts tested, except lettuce. No web blight symptoms were observed on corn or rice, and only minor symptoms were observed on 10-day-old seedlings of wheat. P. aphanidermatum caused severe preemergence damping-off of all dicotyledonous plant species tested but only caused limited seedling size reduction in corn and wheat. Mefenoxam treatment of spinach seed provided complete protection against preemergence damping-off of spinach at both low (0.15 g a.i./kg of seed) and high (0.70 g a.i./kg of seed) rates of application, and the high rate of the application resulted in complete protection against web blight of spinach for 10 to 20 days after planting.

Lettuce drop, caused by the soilborne fungi Sclerotinia minor and S. sclerotiorum, is an important and destructive disease of lettuce. Research trials in Arizona have shown that some fungicides, such as boscalid and iprodione, can reduce disease incidence up to 50 to 60%. Prior research also demonstrated that viability of S. minor and S. sclerotiorum sclerotia was completely arrested after subjection to a respective 2- and 3-week period of continuous flooding in soil during the summer. Widespread adoption of this cultural disease management tool led to concerns in southwestern Arizona about misuse of a precious resource in an arid environment, damage to drainage canals from excessive ground water, and rising water tables adversely affecting lettuce growth. Overall data from current studies, which compared reduction of sclerotia viability after continuous flooding to less intensive flooding of one 8-h period for 1 to 3 weeks or two 8-h periods for 3 weeks, revealed that continuous flooding was not better than 8-h flooding periods for sclerotia of S. minor but was superior for sclerotia of S. sclerotiorum. Because Arizona lettuce growers have to contend with both species of Sclerotinia, continuous flooding is still the best flooding alternative to achieve virtual elimination of viable sclerotia of both pathogens.

This chapter considers Fusarium wilts of garden lettuce, wild rocket and cultivated rocket, lamb's lettuce, chicory, and endive. The sales of such crops have grown rapidly because of changes in consumers' attitude toward them, especially regarding the consumption of ready-to-eat mixed salads. The emphasis in this chapter is on lettuce, which is a crop of worldwide importance. According to the Food and Agriculture Organization of the United Nations, in 2009, more than 1.1 million ha of lettuce (including chicory) were grown commercially in 94 countries. Fusarium wilt was first reported in 1995 as causing root rot on lettuce in Japan. Some 35 years later, a Fusarium wilt was reported on lettuce in the United States in California. Subsequent research showed both wilt diseases to be caused by the same pathogen: Fusarium oxysporum f. sp. lactucae.

Fusarium wilt of lettuce (Figure 1) is caused by the soil-borne fungus Fusarium oxysporum f. sp. lactucae. This particular form of Fusarium oxysporum is known to cause disease and visible symptoms only on lettuce. There are over 100 known “forma speciales (f. sp.)” or “special forms” of Fusarium oxysporum, each causing disease on one to a few specific plant species. For example, Fusarium oxysporum f. sp. spinacia and Fusarium oxysporum f. sp. asparagi are respective pathogens of spinach and asparagus. There are three reported races of Fusarium oxysporum f. sp. lactucae. All three races of the lettuce pathogen are present in Japan, whereas only race 1 is known to occur in the United States (Arizona and California), as well as in Argentina, Brazil, Iran, Italy, Portugal, and Taiwan. The plant pathogenic forms of Fusarium oxysproum can also live on dead plant tissue when living host plants are not available, enabling the pathogens to survive in soil indefinitely. Forms of Fusarium oxysporum that are not plant pathogens also reside in soil, living only on dead plant tissue.

Bell and chile pepper plants are affected by the economically important disease Phytophthora blight, which is caused by the oomycete pathogen Phytophthora capsici. Greenhouse and field trials were conducted to evaluate and compare the ability of nine different fungicides to reduce development of the crown and root rot phase of Phytophthora blight and the resulting chile pepper plant death when applied at 2- and 4-week intervals. Overall, chile pepper plant mortality was significantly decreased in three greenhouse trials with soil applications of fungicide products containing ametoctradin + dimethomorph, cyazofamid, dimethomorph, ethaboxam, fluazinam, fluopicolide, mandipropamid, mefenoxam, and oxathiapiprolin. The same fungicides, excluding mandipropamid and oxathiapiprolin, also significantly reduced overall plant mortality in two field trials. No significant difference was found between 2- and 4-week fungicide application intervals with respect to chile pepper plant survival in any greenhouse or field trial. In general, the degree of reduction in chile pepper plant mortality was lower in field compared to greenhouse trials, probably due to the respective soil surface spray compared to soil drench method of fungicide application used in each instance. Accepted for publication 17 November 2015. Published 30 November 2015.

Phytophthora blight, caused by the oomycete pathogen Phytophthora capsici, is an economically important disease in bell and chile pepper. Fourteen different fungicides were evaluated with respect to inhibition of stem lesion growth on chile pepper seedlings inoculated with mycelium or with zoospores of P. capsici 1 or 3 weeks after treatment of plant foliage and stems or roots. Fungicides containing ametoctradin + dimethomorph and fluopicolide were the most effective among tested products in both experiments across eight trial parameters (inoculum type, inoculation time after treatment, and fungicide application site). Other active ingredients, including acibenzolar-S-methyl, dimethomorph, fenamidone, ethaboxam, mandipropamid, mefenoxam, and oxathiapiprolin, were most effective in reducing stem lesion growth in three to seven of the eight trial parameters evaluated. Compared to nontreated plants, stem lesion inhibition ranged from 84.1 to 100%. Data from these trials demonstrate the comparative effectiveness of tested products under controlled environmental conditions favorable for disease development; however, confirmation of these findings is required in field trials, where plant and environmental conditions will be variable. Accepted for publication 18 September 2014. Published 1 November 2014.

Fusarium wilt of lettuce, caused by the soilborne fungus Fusarium oxysporum f. sp. lactucae, affects all major lettuce production areas in California and Arizona. In trials at UC Davis, we found that lettuce cultivars differ significantly in susceptibility to the disease, with some leaf and romaine types highly resistant under all test conditions. For more susceptible cultivars, disease severity is strongly influenced by inoculum levels and ambient temperature. Management of Fusarium wilt requires an integrated approach that includes crop rotation to reduce soil inoculum levels and the use of resistant cultivars during the warmest planting windows.

Powdery mildew of cucurbits, caused by Podosphaera xanthii (syn. Sphaerotheca fuliginia auct. p.p. (Schltdl.) Pollacci), is a common and often severe disease in most areas of the world. Field trials were conducted with cantaloupe to compare disease management success provided by conventional fungicides and biofungicides having different inherent efficacies and modes of action, when applied alone throughout the treatment period or as components of fungicide application programs. Additionally, the portion of total disease control provided by each component fungicide within selected rotational programs was determined. When applied alone throughout the treatment period, disease severity in 2008 and 2009 compared with nontreated plants was reduced by values of 100, 99.3, and 98.1% by wettable sulfur (Microthiol Disperss), triflumizole (Procure), and quinoxyfen (Quintec), respectively; 83.9, 76.4, and 57.4% by trifloxystrobin (Flint), pyraclostrobin (Cabrio), or azoxystrobin (Quadris), respectively; and 39.8, 31.1, 30.0, and 28.6% by thiophanate-methyl (Topsin M), potassium bicarbonate (Kaligreen), kresoxim-methyl (Sovran), and Bacillus subtilis (Serenade), respectively. Rotational application programs composed of Microthiol Disperss, Procure, and Quintec reduced powdery mildew severity on cantaloupe by 97.5 to 100% in both trials. In comparison, disease reduction of 86.0 to 100% was achieved when the first and third fungicide applications were Quintec or Procure and the second and fourth applications were Cabrio, Flint, Kaligreen, Quadris, Serenade, Sovran, or Topsin M. In field trials designed to elucidate the portion of total disease control provided by each component fungicide within a rotational program, application sequences of Procure, Streptomyces lydicus (Actinovate), Procure, and Actinovate or Procure, Kaligreen, Procure, and Kaligreen resulted in reductions in powdery mildew severity of 69.1 and 78.7%, respectively. In comparison, inclusion of only the two Procure applications brought about a mean disease reduction of 85%, whereas inclusion of only the two Actinovate or Kaligreen applications reduced the level of powdery mildew control to 17.6 and 12.9%, respectively. The usefulness of fungicides with low inherent efficacy as resistance management partners when applied with highly efficacious at-risk fungicides may be questioned; however, the importance of any fungicide as a resistance management partner should be measured by its ability to dilute the selection pressure of the at-risk fungicide and to inhibit the growth of any resistant biotypes that may arise, factors which may not be reflected by its inherent efficacy in controlling disease. Because less efficacious fungicides did not increase the overall level of disease control, encouraging growers and pest control advisors to incorporate them into powdery mildew treatment programs may be a challenge, because resistance management is not their top priority. An ongoing educational effort emphasizing the benefits of resistance management programs with respect to prolonging the effectiveness of single-site mode of action fungicides is essential.

A field survey of mature lemon trees showed an average of 30% of trees with symptoms of brown heartwood rot caused by Coniophora sp. In vivo growth of Coniophora inoculated into branches of different types of citrus (Valencia orange, Marsh grapefruit, Orlando tangelo or Lisbon lemon) on rough lemon rootstock was significantly higher in lemon while Coniophora inoculated into Lisbon lemon wood branches on trees established on rough lemon, volkameriana, macrophylla, Cleopatra mandarin, sour orange or Troyer citrange rootstocks showed no significant differences in growth. Vegetative incompatibility trials from one mature orchard demonstrated that isolates from different trees are incompatible. In vitro fungicide trials showed that only NECTEC paste effectively reduced decay on lemon blocks 15 weeks after inoculation with Coniophora. Field fungicide trials showed that NECTEC P paste as well as the blank paste without fungicides, propiconazole at 10,000 pg /ml, imazalil at 20, 000 ,ug /ml or propiconazole plus imazalil in combination at 10,000 and 20,000 ,ug/ml, respectively, significantly inhibited the advance of fungus 7 mo. after inoculation. A second fungus isolated from brown rot in branches in younger orchards was identified as Antrodia sinuosa, a native decay fungus on conifers in Arizona.

Fusarium wilt of lettuce caused by Fusarium oxysporum f. sp. lactucae continues to spread and cause economic losses in Arizona lettuce fields since the initial discovery of the disease in the state in 2001. Studies were initiated to assess the potential of summer soil solarization and flooding as management tools for Fusarium wilt of lettuce in southwestern Arizona production fields. In microplot studies, lettuce plant growth in soil naturally infested with F. oxysporum f. sp. lactucae that was solarized from 2 to 8 weeks was consistently greater than growth in nonsolarized soil. Growth of lettuce in flooded soil containing the pathogen occasionally was significantly higher than in nonflooded soil; however, the effect on plant growth and health was not as consistent as that recorded for solarized soil. In four trials within a field containing F. oxysporum f. sp. lactucae, the incidence of Fusarium wilt on lettuce sown in soil after solarization was reduced from 42 to 91% compared with disease in nonsolarized plots. There was no significant benefit of a 2- over a 1-month solarization period under the conditions of these trials, where the mean soil temperature at a depth of 5 cm during a 1-month solarization period in 2005 and 2006 was 47 and 49°C, respectively. These findings suggest that soil solarization can be an effective tool for management of Fusarium wilt on lettuce, especially when used within an integrated program in conjunction with existing disease management tactics.

Field experiments were conducted over 2 years in Yuma, AZ, and Holtville, CA, to establish the relationship between soil sclerotium density of Sclerotinia sclerotiorum and the incidence of lettuce drop on different lettuce (Lactuca sativa) types under different irrigation systems, and to determine the efficacy of the biocontrol agent Coniothyrium minitans (Contans) against S. sclerotiorum on crisphead lettuce at varied sclerotium densities under different irrigation systems. There was no significant interaction of irrigation (overhead sprinkler versus furrow) with either sclerotium density or with biocontrol treatment. Lettuce drop incidence was lowest in romaine lettuce compared with crisphead or leaf lettuce at all soil sclerotium densities. There was a significant positive correlation between the sclerotial density and the percent disease incidence. Disease incidence in plots infested with 2 sclerotia/m2of bed was not significantly higher than in control plots regardless of lettuce type. However, plots infested with 40 or 100 sclerotia/m2of bed revealed a significantly higher disease incidence over the control in all lettuce types. A single application of Contans at planting significantly reduced the incidence of lettuce drop in all lettuce types even under high disease pressure. There were no significant differences between recommended (2.2 kg/ha) and high (4.4 kg/ha) application rates of Contans or between one or two applications of the product.

Tecoma capensis (Thunb.) Lindl. (Bignoniaceae, common name: Cape honeysuckle), native to southern Africa, is grown as an ornamental plant in warm regions of the USA. The powdery mildew reported previously from T. capensis in North America was an undetermined Oidium species in Florida. The present report documents the occurrence of the powdery mildew fungus Erysiphe peruviana (Syd.) U. Braun & S. Takam. on T. capensis in Arizona. Accepted for publication 4 January 2010. Published 15 March 2010.

During the life of a citrus planting, the population of Phytophthora pathogens can build to significant levels in orchard soil. A study was initiated to examine the impact of some nonchemical cultural practices on survival of P. nicotianae, the most prevalent Phytophthora sp. in Arizona citrus groves, in soil formerly planted to citrus. In three trials over a 3-year period, P. nicotianae could not be detected at a depth of 10 cm after soil naturally infested with the pathogen was subjected to a dry summer fallow period of at least 31 days in the desert southwest region of Arizona. The mean temperature of soil at this depth during these trials ranged from 37 to 39°C. Furthermore, in two of these trials, after summer dry fallow periods of 38 and 45 days, the pathogen could not be detected at a depth of 15 to 20 cm and was detected in only one of 19 soil samples at a depth of 25 to 30 cm. In comparison, the pathogen was recovered from a high proportion of soil samples subjected to a dry winter fallow period or maintained in the greenhouse and planted with a seedling of citrus, alfalfa, or irrigated without the presence of any plant, where mean temperature of soil ranged from 15 to 30°C. In regions with a hot and dry summer climate, a dry summer fallow treatment of soil after removal of an existing citrus planting and before establishment of a new grove could provide a rapid and relatively inexpensive means of lowering the population of P. nicotianae to virtually nondetectable levels to at least a depth of 30 cm.

Powdery mildew, caused by the fungus Golovinomyces cichoracearum (formerly known as Erysiphe cichoracearum), can develop rapidly in spring lettuce during March and April in western Arizona, as the crop nears maturity, when moderate to warm temperatures and dry environmental conditions prevail. The first signs of disease can occur as early as December or January. Successful chemical control of pow dery mildew requires the presence of an e ffective fungicide on plants before disease onset, followed by successive applications of materials to maintain disease control until harvest. A field trial was conducted in 2007 to test and compare the efficacy of some registered as well as new fungicides, applied alone or in a rotational treatment program, for management of powdery mildew. Foliar applications of treatments were made Jan 24, Feb 1, Feb 16 and Feb 28, 2007. Among treatments, the degree of powdery mildew control ranged from virtually c omplete to minimal; however, a ll treatments significantly r educed disease severity compared to untreate d plants. Treatments that reduced the severity of powdery mildew more than 90% compared to untreated plants included Procure alternated w ith Quintec, Quintec, Microthiol Disperss, and Procure alternated w ith Mic rothiol Disperss. Yiel d loss due to reje cted le ttuce heads usually would begin to occur when the powdery mildew disease rating exceeds 2.0. Fungicide treatments that kept disease severity below this level (in addition to the treatments already listed) include V-10118, Procure alternated with Quadris, Procure, Switch, Forum + Cabrio, Endorse, Maneb + Reason, and Endura. This trial was initiated as a combined downy and powdery mildew trial; therefore, some of the products were placed in the test specifically for downy mildew. Due to low humidity levels and no rainfall during the trial, no downy mildew developed; however, some of these downy mildew fungicides, such as Forum, Maneb, and Reason, significantly suppressed powdery mildew. Phytotoxicity symptoms were not noted on lettuce for any of the materials tested.