Preprint · bioRxiv 2026  ·  poster, ICMA-X 2026

When should a pathogen strike? Navigating the landscape of birth pulses and mating season

Bhargav D. Srinivasan, Emily L. Bruns, Samuel V. Hulse

Bruns Lab · Department of Biology, University of Maryland, College Park

The question

Hosts offer a pathogen two chances to spread, at different times of year.

The birth pulse brings few hosts, but young ones that are highly susceptible.
The mating season brings many adult hosts in contact, but they resist infection better.
Anther smut (Microbotryum) replaces pollen with spores. Pollinators carry them to flowers, and wind carries them to seedlings.
In the field, infected Silene flower a few days early in S. latifolia, but up to four weeks early in Himalayan species.
1.5:1
juvenile : adult susceptibility, S. latifolia
0:1
juvenile : adult susceptibility, D. pavonius
The model  ·  Silene latifolia and its anther-smut fungus

An age-structured model with seasonal germination and flowering.

Seasonal windows

Germination and flowering each run about 98 days a year. We move their midpoints through the year.

Sterilizing infection

Infectious flowering plants infect all stages, juveniles most of all. Infected plants make no seeds.

Invasion analysis

A rare mutant flowering time invades if its yearly growth rate (Floquet exponent) is positive.

Result 1  ·  healthy and infectious flowering fixed at mid-year; germination moved across the year

Disease peaks when germination overlaps flowering.

Less disease when germination avoids flowering
0–0%
Largest cut under slow maturation, when juveniles stay susceptible longest.
Peak germination midpoint
flowering midpoint 182.5day 189
Faster maturation raises prevalence
infected juveniles start flowering sooner↑
Result 2  ·  healthy flowering fixed at mid-year; infectious flowering moved across the year

Prevalence peaks when infectious flowering follows the juveniles.

3–5 wkfrom the juvenile peak to the best infectious flowering time
day 64–76best time for spring germination, earlier with faster maturation
day 251, 264best times when germination is at or after flowering
Result 3  ·  invasion analysis, S. latifolia parameters, 18 germination midpoints

One flowering time is favored, about seven weeks after germination.

ESS after the median germination day, at every germination time44–60 d
The ESS jumps where germination splits into a spring flush and a summer cohort.
Result 4  ·  germination midpoint day 119, split between spring and summer cohorts

Fast-maturing or highly susceptible juveniles let two strategies coexist.

D. pavonius, estimated parametersdays 98 + 187
S. latifolia, juveniles maturing twice as fastdays 93 + 253
S. latifolia, juveniles ≥ 6× as susceptible as adultscoexist