Biological Characteristics that Make Broomrape a Threat to California Crop Production Systems

 

 

In a previous article we gave a general background of branched broomrape (Phelipanche ramosa), a parasitic weed which was the focus of a $1.5 million eradication effort four decades ago in California, and now a re-emerging threat to California processing tomato (link, Figure 1). The threat posed by branched broomrape is different than most agricultural weeds due to its unique life cycle. Understanding its biology is an imporant first step in providing strategic and sustainable control of this weedy broomrape in California crop production systems.

Branched broomrape is an obligate parasite, meaning that all stages of its life cycle, right from germination to seed production depend entirely on the presence of a suitable host plant and a relatively narrow range of environmental conditions.

In California these conditions typically are met within mid-spring to late summer. Before germinations branched broomrape seeds goes through a preparatory stage called pre-conditioning period to break seed dormancy, under a moist and warm field condition with daily temperature of 60 to 70 ^oF for at least 5 to 7 days. This pre-conditioning period often coincides with early tomato planting in many parts of California, that is, mid-spring (March to April). Then, after tomatoes are transplanted, the pre-conditioned broomrape seed can germinate in response to a signaling stimulant (strigolactone) released from the host plant roots (Figure 2). If the field conditions remain conducive, there can be multiple flushes of germination within a single season (Figure 3); however, in the absence of a signal from a suitable host these pre-conditioned seeds may once again become dormant. The ability of the seed to germinate reduces gradually as the field becomes dry, and the possibility of seed germination can be very low when average daily temperature is greater than or equal to 85 ^oF, during late summer.

After germination, the radicle (immature primary root) of the broomrape seedling grows a few millimeters in length to encounter a host root. If it does not encounter and attach to a suitable host, it is likely to exhaust its energy reserves within a few days. Interestingly, not all plants that release stimulants for branched broomrape germination allow the penetration and connection of this parasite to their system. These plants are called false hosts or trap crops and these may include alfalfa (Medicago sativa),cowpea (Vigna unguiculata), green pea (Pisum sativum) and flax (Linum usitatissimum). These trap crops can be used to promote germination of branched broomrape without supporting their survival; thereby, reducing populations of the weed in the soil seedbank.

Following a successful attachment to a host plant such as tomato, the radicle develops into a specialized modified root called haustorium, a plant organ common to all parasitic plants.

The haustorium fuses into the vascular system of the host root and serves as the bridge for extraction of water and nutrients from the host plant. Once connected to a host plant, broomrape grows rapidly, forming a tubercle underground (Figure 4). The knowledge of the period at which tubercle is being formed is very useful in controlling broomrape. Multiple shoots develop from a single tubercle (Figure 5) and emerge above the soil surface, then grow to 5 to 15 inched tall stalks (Figure 6). The shoots are wrapped with alternate bracts but completely lack leaves and chlorophyll (Figure 6). This lack of leaves and chlorophyll is an indication that branched broomrape lacks the ability to synthesize food by itself, and that weed control inputs such as foliar herbicides have no platform for activity in the parasite.

Once it emerged above soil surface, branched broomrape rapidly proceeds to the reproductive stage; flowering within 3 to 7 days after

emergence. Branched broomrape flowers are spike-like, irregular, bisexual and usually pale white to purple in color. The petals of the flower are merged, tubular with an upper and lower lip (Figure 7). The carpels are usually united to form a single chamber on the upper part of the flower; this chamber matures as a capsule with thousands of very tiny seeds (0.2 to 0.4 mm), smaller than a grain of sand (Figures 8). Seed production can occur as quickly as two weeks after flowering and a mature broomrape plant can produce hundreds of thousands of tan or brown-colored seeds. The seeds can remain dormant and viable for many years (> 20 years) in soil. These tiny seeds are easily spread through wind, water, animals, contaminated farm implements and produce. Branched broomrape life cycle from seed germination to seed production is within the March to early September growing season of processing tomatoes in California.

Conclusion

Effective control of broomrapes is difficult, largely due to its unique biology and complex life cycle. As indicated above, most of the broomrape life cycle occurs below the soil surface, which makes it difficult to detect and control before it causes damage to the host plant. The short time period between emergence and seed dispersal also makes detection and control difficult, while the absence of chlorophyll and photosynthesis limits potential herbicide target sites and complicates chemical management of the weed. The hard-to-detect, abundant, tiny seeds and ability of the seeds to remain viable for decades, promotes the spread and persistence of branched broomrape in crop production systems. Thus, effective management of broomrape requires a long-term integrated approach that involves sound understanding of its localized and general biology. Herbicide programs for management of a related species, Egyptian broomrape have been developed in other countries based on understanding of the parasitic weeds lifecycle. University and industry research is ongoing in California to develop similarly-effective practices for the California processing tomato industry (link).