Sampling Soil and Roots in Citrus Groves: Principles and Methods Larry Duncan UF, IFAS, CREC Lake Alfred Observation Measurement Why sample? How many? When and where? Then what? Observation Never miss opportunities to examine the root systems of trees. Conventional APS, microjet APS, drip Schumann APS trial Auburndale Severely affected by HLB by year 4. Exacerbated by other pests? Measurement What is there? Are there enough to be damaging? How can we know what is there with reasonable certainty at an affordable cost? (sample optimization) . From the APS survey, how confident can we be that pests caused greater damage to roots in the APS plots than in the conventional plots? Confidence interval – if we had sampled this block the same way 100 times, 95% of the 100 mean estimates would be in the range shown by the error bars. P=0.003 If there is actually no difference between these treatments in how much pests damaged the roots, we could expect to find this difference in our sample results just 3 times in one thousand trials. Confidence interval = x ± 2 s / √n P=0.003 Length of error bars depends on the natural variation that occurs among samples and the number of samples taken. This is an example of what we P=0.62 might find if we had sampled just 6 instead of 16 trees in each treatment. Optimizing sample size P=0.003 Standard error = d x = s / √n where d=some proportion of x √n = s / dx n = (s / dx)2 So if we know the likely mean and P=0.62 standard deviation we can estimate an appropriate sample number. Optimal sample numbers have been estimated for many insect and nematode pests and most methods are based on some variation of this formula: b-2 2 Number samples = zἀ/2ax /(0.5 CI/x) Which is based on a remarkable relationship described as Taylor’s Power Law. All organisms display this property. The sample variability depends on the population density. To estimate a sample size that will deliver a certain accuracy we need only to know the expected mean and variance. Taylor’s Power Law allows us to select a mean density (e.g., economic threshold) and predict the variance and an optimal sample size. First principle of sampling. Sample size decreases as population size increases. We can increase sample accuracy by sampling when and where population density is highest (e.g. mid-way between trunk and dripline during spring or late autumn for most nematodes. Stratify site base on properties affecting organisms). Second principle of sampling. Organisms are aggregated, clumped. Use a systematic pattern, don’t sample randomly. The most effective sampling pattern is one in which each sample is equally distant from the others. This is what a clump of potato cyst nematodes looks like. The dimensions of the clumps are related to the population density. By knowing the dimensions of the smallest detectable clump sampling plans were Development and evaluation of sampling methods for fields with infestation foci of potato cyst nematodes. Been and Schomaker (2000) Phytopathology. developed that detected The demarcation of cyst nematode infestations in field for seed potatoes using a monte carlo approach. Schomaker and Been (2010) OEPP/EPPO Bulletin. this nematode with 95% reliability. Polk County Indian River County <DRW H. indica 96% sand >DRW 52% mortality (35-80%) 88% sand 42% EPN (29-78%) 28% mortality 14% EPN H. zealandica Osceola County S. diaprepesi >>>DRW 53% sand 16% mortality 3% EPN H. indica H. indica A zigzag pattern is an effective method of spacing soil samples at regular intervals. The figure is based on the spatial pattern of soybean cyst nematode in a Missouri soybean field. APS Press, 1999 Two samples of 15 cores each (12 inch depth, 2-5 acres) adequate to estimate root and nematode density. Stratify area based on physical properties likely to affect target. Systematic, not random sampling pattern. Under canopy, midway between trunk and dripline. Best to sample in spring and late autumn. Break 1 20 The HLB Era: “manageable problems have become intolerable” Larry Duncan CREC D. abbreviatus Citrus root weevil refers to a complex of species. Diaprepes abbreviatus is the most damaging to citrus, but blue green weevils (Pachnaeus litus) are abundant and damaging in some parts of Florida. Little leaf notcher P. litus (Artipus floridanus) is not thought to be economically important. A newly introduced species, Sri Lanka weevil (Myllocerus undatus) looks similar to little leaf notcher and its economic importance is unknown. A. floridanus M. undatus Diaprepes abbreviatus life cycle Eggs Adults Neonates Pupae Larvae + Phytophthora The abundance of weevil populations and their damage to trees varies regionally 0.6 0.25 Weevils are occasional Flatwoods -1 Ridge 0.20 and sporadic pests on week 0.5 -1 0.15 the central ridge -1 0.10 0.4 0.05 Weevils are major and Weevils trap Weevils week 0.00 chronic pests in the -1 Lake DeSotoHendryn River Polk 1Polk 2 0.3 India flatwoods 0.2 Consequently, another consideration is that Weevils trap Weevils 0.1 weevil management should be considered 0.0 to be habitat specific MJJASONDJFMAMJJASONDJFMAMJJASON 2001 2002 2003 Futch et al., Fla. St. Hort. Proc. 2004 Typical seasonal pattern of adult weevil abundance in citrus orchards In this Polk County grove, weevils emerged from soil in April-May each year (with a 2nd Fall emergence in one year). The timing varies from grove to grove. So monitoring weevil abundance is another important managemet tactic. Adult weevil management should be coordinated to follow peak adult emergence by treating with adulticides and ovicides. Duncan et al., 2001, Environmental Entomology Citrus root weevils Why so hard to manage? Basically… Modern pesticides have little residual activity Different weevil stages live in both the tree canopy and the soil. They serve as source of new individuals to replace those killed by management practices Damage to roots is cumulative. Yearly damage by just a few larvae eventually kills trees. The fact that weevils comprise part of a pest-disease complex is the basis for an important management tactic… While there are currently no rootstocks that are resistant or tolerant to weevils, they do exist for Phytophthora control. Diaprepes- Phytophthora complex evident on the sour orange trees, but not the Swingle citrumelo resets, in this DeSoto County grove. Symptoms of the pest-disease complex on roots of sour orange Rootstocks for central ridge (Phytophthora nicotianae) Swingle citrumello Carrizo citrange C-35 C-32 Rootstocks for flatwoods (Phytophthora palmivora) Cleopatra mandarin US-802 US-897 Relationship Between Tree Decline and Phytophthora Incidence Chemical Treatment, r2 = 0.78 Cleo 60 No Treatment, r2 = 0.65 50 40 C-22 C-32 30 Cleo 20 Swingle C-35 C-22 10 C-32 C-35 Percent Tree declinePercent Tree 0 Swingle 10 15 20 25 30 35 40 45 50 Percent Phytophthora Incidence Plot I, Treated, 2005 C22 C35 Swingle C32 Cleo Plot 2, Untreated, 2005 C32 C22 Swingle Cleo C35 Weevil management Rootstocks Regional considerations Cultural practices Soil drainage pH Physical barriers Insecticides (monitoring weevil abundance ) EPNs Management of soilborne insects Diaprepes Task Force website has biliography of all research papers, general information and dichotomous management key for 8 common grove situations Management of soilborne insects Each of 8 common scenarios provides links to resources from which recommendations were derived Management of soilborne insects Each of 8 common scenarios provides links to resources from which recommendations were derived Cultural practices Soil drainage is critically important because weevil abundance and therefore root damage is greatest in wet soil. Trees stressed by wet soil are less tolerant of weevil damage. Some natural enemies of weevils are less abundant in wet soil. S. diaprepesi H. indica EPNs EPNs Low area of Diaprepes- infested grove Campos-Herrera et al., 2013 Soil Biology and Biochemistry Cultural practices Low pH High pH Soil pH is critically important because many rootstocks are less tolerant of stress in 1.0 ) high pH soil (>6.5) and 3 0.8 because some important 0.6 natural enemies of 0.4 weevils are intolerant 0.2 of high pH soil. (mg/cm density root Fibrous 0.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Soil pH Graham et al., 2013. Plant Disease Low300 pH High pH Cultural practices Raised natural pH 250 Soil pH is critically pH 5.5 important because some 200 pH 7.0 entomopathogenic 150 nematodes are intolerant of 100 high pH soil, whereas some 50 plant parasitic nematodes 0 are favored by high pH. 180 Steinernema diaprepesi Reduced natural pH 1200 160 140 1000 soil 120 -3 800 100 Recovered IJ 80 600 60 400 40 200 20 Citrus nemas x 100 cm Citrus x nemas 0 0 Ck Ad Ca Control Biocontrol Manure Low pH Treatment Cultural Practices Landscape fabric can be installed as a barrier to prevent larvae from entering soil and adults from exiting soil McKenzie et al. 2001 Duncan et al 2008 The integrity of these Neonate barriers was maintained for 5- larva 6 years. Herbicide savings equaled the fabric cost. Typical effect of landscape fabric mulch on tree growth 3500 Bare soil Landscape fabric In an experiment measuring 3000 Culture P=0.001 effects of Advanced Citrus Mulch P=0.001 CxM P=0.79 Production System (ACPS) versus conventional citriculture (CC), we also installed landscape 2500 fabric under some trees. ACPS trees were 25% larger than CC trees, but only 9% 2000 larger than mulched CC trees. Trunk cross-sectional area (mm) area cross-sectional Trunk 0 Mulched ACPS trees were 35% Conventional ACPS larger than conventional trees.