4.1. Biological Control in the Field
From a commercial point of view, the use of
Bacillus-based preparations has been widely demonstrated for efficiency controlling multiple field fungal pathogens
[84]. However, research for the biocontrol of lupin anthracnose is scarce. Studies conducted in the field with anthracnose natural inoculum pressure showed that all
B. subtilis treatments reduce the area under disease progress curve (AUDPC) compared with controls that showed large stem lesions, accompanied by necrotic tissue 90 days after sowing (DAS) and abundant sporulation, and in some cases dead plant from 105 to 120 DAS
[88]. Lupin anthracnose can crack the stem at the site of infection (
Figure 1A); multiple lesions can lead to stem twisting, or to complete stem collapse
[89][30]. The AUDPC reduced with sequential applications of antagonists 1×10
9 UFC/mL, every two weeks, compared to the AUDPC of untreated control in susceptible cultivars (
Figure 4). In general, treatments with bacterial suspensions result in healthier stems, leaves, and pods, which are agronomic characteristics of lupin linked to production
[88]. The significant reduction of AUDPC may be due to
B. subtilis having a cascade effect on the different components of the disease triangle, that is, on the environment, the host plant, or the pathogen. Therefore, it can affect the onset and progress of the disease and also turn on resistance genes in the host
[90].
B. subtilis induces disease resistance in the plant
[82][84][85], or in turn promotes plant growth, a fact that makes it easier for plants to control pathogenic infections
[79][81][86] or suppress diseases based on systemic resistance
[82][83][84][86].
B. subtilis have also shown to control ginseng anthracnose,
Colletotrichum panacicola, where leaf lesion diameter was not significantly affected compared with the fungicide treatment
[91]. The protection mechanism of
B. subtilis was associated with reduction of the incidence and with the initial processes of the infection cycle such as spore germination, appressorium formation, and penetration
[90].
Figure 4. The area under disease progress curve (AUDPC) of anthracnose caused by C. lupini on the Andean lupin susceptible cultivar I.450 Andino. Natural pressure of inoculum represented as the control (Ck ) and the AUDPC by effect of Bacillus subtilis Ctpx-S1 , Ctpx-S2 , and Ctpx-Z3 . AUDPC calculates scores for lupin anthracnose severity on the 1-to-6 scale [92]. Pooled data from the 2015 and 2019 growing seasons at El Chaupi, Cantón Mejía, Ecuador, each point at disease onset plus 90, 105, and 120 days. Source: [88].
A
B. subtilis strain selection program generally begins with screening under con-trolled laboratory or semi-controlled greenhouse conditions; however, it is difficult to predict how the bacteria will respond when released into the natural environment
[93]. The native bacterial population on the phylloplane was estimated to confirm its protectant effect. Viable cell concentrations of
B. subtilis were reduced in approximately 2.0 logarithm after 15 days of the first application on Andean lupin (
Figure 5)
[88] or reduced by 50% in strawberry leaves in the open field after 8 days of application
[93]. Environmental stresses such as intense sunlight, dryness, or high temperature could reduce initial colonization
[88][90], or rainfall could easily remove the bacterial population from the plant surface in several hours
[93]. However, the population of
B. subtilis will vary to some degree naturally once it has been established in the field. The average of native
B. subtilis population on the phyllosphere of lupin remains stable at approximately 7.0 LOG after successive two-week-sprays for two months, which suggests that after the population has become stable, it can resist harsh environmental conditions (
Figure 5)
[88].
Figure 5. Population of Bacillus subtilis Ctpx-S1 , Ctpx-S2 , and Ctpx-Z3 recovered from the phyllosphere of lupin cultivar I-451 Guaranguito during 2015 and 2019 growing seasons. First spray at disease onset and fortnightly for eight weeks. Colony counted and results expressed as CFU B subtilis per gram of lupin leaves. To improve homogeneity of variances, data of bacterial concentration were log 10-transformed (log10 CFU/g). Each point represents the mean ± SD of four independent repetitions of 10 g leaves. Source: [88].
The total viable epiphytic population on the phylloplane can be determined by a serial dilution plating method considering that bacteria can colonize stomata, trichomes, vein endings, cell wall junctions, or even be beneath the leaf cuticle
[94]. In the work of Falconi et al.
[88], lupin stems and leaves sprayed with bacterial antagonists were consistently protected from natural infection of
C. lupini fungi. Control was evident in a significant reduction in the AUDPC and suggests that native
B. subtilis are more likely the cause of reduction in anthracnose severity. In addition, plants release organic compounds, such as sugars, organic acids, and growth regulators
[95] that can stabilize cell concentration of
B. subtilis due to nutrient availability
[96]. Thus, when bacterial antagonist adhered, invaded, and survived on flowers, they effectively prevented the pathogen from colonizing the flowers
[93]. Addition of 0.5% unrefined sugar to
B. subtilis suspensions provides an initial food supply and increases adherence and colonization of bacteria in cocoa pods, improving monilia pod rot control
[96][97]. Other studies show that a single inoculation of
B. pumilus reduced banana sigatoka,
Mycosphaerella fijiensis, by 33.6% and delayed progress of the disease by 21–28 days compared with the control
[98]. Lower AUDPC values in comparison with the control indicates reduction of the initial inoculum or reduction of pathogen on the plant
[98]. By associating prior findings
[36][37][38] with the field observations where infections and disease severity were reduced, it can be speculated that biocontrol by native
B subtilis resulted not only from damage of cellular structures (hyphae, mycelium, spores) of the pathogen, as occurred in the in vitro studies, but also by induction of systemic resistance (ISR) and promotion of plant growth mechanism (PGM). An innovative strategy for the integrated management of anthracnose in lupin could involve the activation of ISR by
B. subtilis. Future studies could demonstrate more efficient use of these native strains and perhaps allow the establishment of biocontrol strategies.
4.2. Chemical Control
As there are no resistant varieties of Andean lupin and in the absence of any accurate method of controlling the disease, chemical control has been recommended as the most effective measure to reduce the spread of the disease. Synthetic fungicides reported for control of lupin anthracnose in the Andean zone and around the world are listed in
Table 3. Seed treatment with fungicides reduces transmission of the pathogen from seed coat to seedling, but it does not provide complete control
[99]. The synthetic fungicides generally recommended for controlling anthracnose disease are based on copper compounds, dithiocarbamates, benzimidazole and triazole compounds, manganese ethylenebisdithiocarbamate (Maneb)
[99][100][101], and carbendazim
[100], although the use of fungicides has been found ineffective under severe disease outbreak
[99]. Newer chemicals like strobilurins (e.g., azoxystrobin, pyraclostrobin) have also been used for lupin anthracnose management
[102][99][103]. Different classes of fungicides have specific modes of action and also differ in the duration of disease control. Farmers should take prevailing environmental conditions into consideration in their choice of fungicides. Rotation of two or more different classes of fungicides is highly recommended for increasing the chance of better protection against the disease in the fields
[101][103][104][105]. Applications of Boscalid + Piraclostrobin or Azoxystrobin + Difeconazole significantly reduced the severity and infection in seed and significantly increased grain production
[102].
Table 3. Fungicides and bioproducts reported for the control of lupin anthracnose.
ctive Ingredient |
Chemical or Biological Group |
Target Site * |
FRAC Code ** |
References |
Iprodione |
dicarboximides |
MAP/Histidine-Kinase in osmotic signal transduction |
E3/2 |
[99][100] |
Procymidone |
dicarboximides |
MAP/Histidine-Kinase in osmotic signal transduction |
E3/2 |
[99][100] |
Carbendazim |
benzimidazoles |
ß-tubulin assembly |
B1/1 |
[99][100] |
Difeconazole |
triazoles |
C14- demethylase in sterol biosynthesis |
G1/3 |
[99] |
Copper |
inorganic |
Multi-site contact activity |
M 01 |
[99] |
Mancozeb, Maneb |
dithiocarbamates |
Multi-site contact activity |
M 03 |
[99][100][101] |
Captan |
phthalimides |
Multi-site contact activity |
M 04 |
[99] |
Folpet |
chloronitriles |
Multi-site contact activity |
M 05 |
[99] |
Clorotalonil |
chloronitriles |
Multi-site contact activity |
M 05 |
[99][101] |
Dithianon |
quinones |
Multi-site contact activity |
M 05 |
[103][104] |
Ciprodinil + Fludioxonil |
anilino-pyrimidines + phenylpyrroles |
Multi-site contact activity |
M 09 |
[104] |
Fenhexamid + Tebuconazole |
hydroxyanilides+ triazoles |
3-keto reductase, C4- de-methylation + C14- demethylase in sterol biosynthesis |
G3/17 + G1/3 |
[104] |
Azoxistrobin |
QoI-fungicides |
Complex III: cytochrome bc1 |
C3/11 |
[99][100][101] |
Pyraclostrobin + Boscalid |
methoxy-carbamates + pyridine-carboxamides |
Complex III: cytochrome bc1 at Qo site + complex II: succinate-dehydrogenase |
C3/11 + C2/7 |
[102] |
Azoxystrobin + Difeconazole |
QoI-fungicides + triazoles |
Complex III: cytochrome bc1 at Qo site + C14- demethylase in sterol biosynthesis |
C3/11 + G1/3 |
[102] |
B. subtilis |
microbial |
Multiple effects antibiosis, membrane disruption byfungicidal lipopeptides, lytic enzymes, induced plant defense |
BM 02 |
[38][84][106] |
Effective control through the use of chemical fungicides is possible by timely application during the critical period favorable for the onset of the disease
[99][100][101][103][104]. Generally, fungicides should be applied for seed disinfection
[99], at young seedling emergence, including early flowering or full bloom, and early pod formation to restrict the entry of the pathogen to the plant system
[99][100][101][103][104].
Rotating fungicides with different FRAC codes can help delay development of lupin anthracnose resistance to fungicides. Code numbers on fungicide labels, called “FRAC” groups, can help to develop chemical rotations that delay fungicide-resistance (Table 4). The optimal application time is when the C. lupini population is small and contained, when weather conditions make the pathogen susceptible to fungicide, or when lupin is at the most susceptible growth stage (Figure 6).
Figure 6. Anthracnose susceptible phenological stages of lupin.
B. subtilis for control of lupin anthracnose is also listed in
Table 3. Application of sus-pensions based on
B. subtilis [38][88][106] into a rotation with foliar fungicide containing mancozeb, chlorothalonil, or azoxystrobin
[102][107][108] at sowing, plant emergence, early flowering, and early pod formation can better prevent transmission of the pathogen to the seedling and suppress anthracnose in the field. Using a biocontrol agent can reduce damage to human health and the environment.
4.3. Control Volunteer Lupins and Alternative Hosts
Lupin production in the tropical Andean highlands varies in the amount planted, which in general follows local rainfall patterns and prevailing prices. The highland tropics are also the home of several alternative hosts for anthracnose, including tamarillo (
Solanum betaceum), that are geographically grown in neighboring lands with lupin and give the pathogen more opportunities to reproduce and survive
[109]. Other studies also showed that
C. lupini was able to cross-infect other host plant and
Colletotrichum species from soybean also cross-infected lupin with varying degrees of virulence
[110]. The plasticity of the interaction of
Colletotrichum spp. with different hosts is significant. In addition, several species of wild lupin, such as lupin afelpado (
L. alopecuroides), lupin rastrero (
L. sarmentosus), miniature lupin or allpa chocho in Quechua language (
L. microphyllus), or Quito’s chocho (
L. pubescens)
[111], grow in tropical highland areas and thereby act as a reservoir for pathogen growth during drier periods
[112].
L. cosentinii can act as another source of infection in Western Australia, where control of naturalized populations is an important component for disease management
[13]. In the Andean areas, high levels of inoculum could come from other susceptible cultivated or wild lupins and other alternative hosts in the surrounding areas; consequently, sanitation at the field or farm level is very important for anthracnose management
[109].
4.4. Ontogenic Resistance
The ability of whole plants or plant parts to resist or tolerate disease as they age and mature is called ontogenic or age-related resistance. Levels of resistance that develop during aging of plant tissue may greatly reduce eventual disease severity and may even lead to escape from infection or immunity
[113]. In lupin, anthracnose resistance is not equally expressed at all developmental stages. An ontogenic model was developed for three lupin species—
L. angustifolius,
L. luteus, and
L. albus—and integrated into a decision support system for anthracnose management. The beginning of flowering and full bloom represent growth stages with high susceptibility
[114]. Based on a series of in vivo assays involving Andean lupin genotypes, early cotyledonal stages (2–3 leaves) and the beginning of flowering (9–11 leaves) were susceptible to anthracnose, whereas the vegetative development stage (6–8 leaves) was resistant
[92]. Cotyledonal and flowering stages
of L. mutabilis were confirmed as susceptible to anthracnose through spectroradiometry and unmanned aerial vehicles
[115]. This plant response of susceptible genotypes of Andean lupin in Ecuador
[3] reflects the current situation of resistant and susceptible cultivars of
L. angustifolius in Australia
[99][116], where epidemics of lupin anthracnose progress more slowly in plots of intermediate-aged plants and dramatically increase during flowering. Some stage-specific resistance genes might not be expressed in early and late lupin developmental stages
[92]. Targeting disease susceptibility during the life cycle of the Andean lupin plant will contribute to development of a more viable management option for production in areas of high anthracnose risk in Ecuador and maybe to more efficient lupin anthracnose management around the world. Early growth, beginning of flowering, and early pod formation
[92][99][114][115] are the stages when lupin is most susceptible to anthracnose. An integrated ontogenic plan can be used during these major plant susceptibility periods for scheduling application of biological or chemical treatments.
4.5. Integrated Methods at Critical Phenological Stages
No single management technique has been found to efficiently control lupin anthracnose, therefore integration of techniques is essential. The first critical step is to reduce infection in seed (
Figure 5) by using dry heat
[33], UV-C radiation
[34], or UV-B radiation
[35]. Further protection can be added by combining physic treatments with other techniques, such as biological
[36][37][38][83][87] or chemical control
[102][99][100][101][103][104]. If one of the physic treatments of seed is used for high rainfall regions, it should be supplemented with foliar sprays to suppress pathogen dispersion in the field and subsequent infections. Application of fungicides on flowering and podding stages reduced incidence and increased yield of susceptible and resistant cultivars grown in high-risk areas
[99].
Applications of
B. subtilis theoretically prevent the establishment of the pathogen, interrupt its development, promote plant growth, and induce acquired resistance in lupin
[38]. Because local results obtained under field conditions are consistent
[88], bacterial suspensions at 1 × 10
9 UFC/mL should be applied prior to pathogen infection, at sowing time (
Figure 6). Success in controlling lupin anthracnose disease can be achieved with a sequential two-week application
[88]. Shorter interval applications of
B. subtilis are often recommended during the rapid growth stages of the plant to obtain a commercially acceptable product
[106]. Special attention should be given to cotyledonal, early flowering, full bloom, and pod-filling stages (
Figure 6) because lupin has been shown to be more anthracnose susceptible at these phenological stages
[90][92][114][115].
Other technological components are needed to further reduce the use of synthetic pesticides. For this, it is necessary to develop
B. subtilis at a semi-industrial or industrial level or to use the pathogen-inhibiting substances extracted from bacteria in rotation plans with chemical products. These rotation plans would include the interaction of
B. subtilis-based biopesticides with protective and systemic fungicides to determine dose, frequency, and times of application
[117]. A low-cost medium for
B. subtilis lipopeptide production is available
[118], and a spray drying formulation
[119][120] has been developed to expand survival and reproduction of the antagonist under field environmental conditions. A plan that includes the rotation of biological fungicides with chemicals and their application during the most susceptible plant growth stages will greatly minimize infection in the field and reduce losses at harvest for a more sustainable production of Andean lupin, until resistant varieties are developed.