While the potato spindle tuber viroid (PSTVd) variant, PSTVd-Dahlia (PSTVd-D or PSTVd-Dwt) induces very mild symptoms in tomato cultivar ‘Rutgers’, PSTVd-Intermediate (PSTVd-I or PSTVd-Iwt) induces severe symptoms. These two variants differ by nine nucleotides, of which six mutations are located in the terminal left (TL) to the pathogenicity (P) domains. To evaluate the importance of mutations located in the TL to the P domains, ten types of point mutants were created by swapping the nucleotides between the two viroid variants. Bioassay in tomato plants demonstrated that two mutants created on PSTVd-Iwt at positions 42 and 64 resulted in symptom attenuation. Phenotypic and RT-qPCR analysis revealed that mutation at position 42 of PSTVd-Iwt significantly reduced disease severity and accumulation of the viroid, whereas mutation at position 64 showed a significant reduction in stunting when compared to the PSTVd-Iwt infected plant. RT-qPCR analysis on pathogenesis-related protein 1b1 and chalcone synthase genes showed a direct correlation with symptom severity whereas the expansin genes were down-regulated irrespective of the symptom severity. These results indicate that the nucleotides at positions 42 and 64 are in concert with the ones at positions 43, 310, and 311/312, which determines the slower and stable accumulation of PSTVd-D without eliciting excessive host defense responses thus contributing in the attenuation of disease symptom.

A healthy, latently infected, diseased (HLD) plant model for botanical epidemics was defined for tomato bacterial canker (TBC) caused by the pathogenic plant bacteria, Clavibacter michiganensis subsp. michiganensis (Cmm). To estimate the infection probability parameter, inoculation experiments were conducted in which it was assumed that infection is transferred to healthy plants through contaminated scissors used to cut symptomless infected plants. The approximate concentration of Cmm in symptomless infected plants was 1 × 106 cells/mL, and the probability of infection of healthy tomato plants was approximately 0.75 due to cutting with scissors soaked in a cell suspension of Cmm at 1 × 106 cells/mL. Three different HLD models were developed by changing some parameters, and the D curve calculated by the developed HLD model A was quite similar to the curve of the proportion of diseased plants observed in fields that had a severe disease incidence. Under a simulation of disease incidence using this model, the basic reproduction number (R0) was 2.6. However, if the infected scissors were disinfected using ethanol, R0 was estimated as 0.3. The HLD model for TBC can be used to simulate the increasing number of diseased plants and the term of disease incidence.

A pathogen, healthy, latently infected, infectious, and diseased plant (PHLID) model for botanical epidemics was defined for tomato bacterial canker (TBC) caused by the pathogenic plant bacteria, Clavibacter michiganensis subsp. michiganensis (Cmm). First, the incubation period had to be defined to develop this type of model. To estimate the parameter of incubation period, inoculation experiments were conducted in which it was assumed that infection is transferred to healthy plants by cutting with contaminated scissors after cutting infected plants with early symptoms or symptomless. The concentration of Cmm was increased over 1 × 106 cells/g plant tissue at 20 cm away from the inoculated point on the stem 10 days after inoculation, and then the approximate incubation period of TBC in symptomless infected plants was defined as 10 days. The developed PHLID model showed the dynamics of diseased plants incidence and fitted the curve of the proportion of diseased plants observed in fields well. This model also contains the factors of pathogen and disease control, and it was able to simulate the control effects and combined two different control methods, which were the soil and scissors disinfections to prevent primary and secondary transmissions, respectively. Thus, this PHLID model for TBC can be used to simulate not only the increasing number of diseased plants but also suppressing disease increase.